THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

FROM  THE  LIBRARY  OF 

PROFESSOR 
CARL  COPPING  PLEHN 

1867-1945 


HANDBOOK 


OP 


NATURAL   PHILOSOPHY, 


FOR 


SCHOOL    AND    HOME    USE. 


BY 


W.   J.    ROLFE, 

FORMERLY   HEAD   MASTER  OF  THE   HIGH   SCHOOL,   CAMBRIDGE,   MASS. 


AND 


J.   A.    GILLET, 


PROFESSOR    OF     MATHEMATICS     AND     PHYSICS     IN     THE     FEMALE     NORMAL     AND 
HIGH    SCHOOL    OF    THE    CITY    OF    NEW    YORK. 


SECOND   EDITION,    REVISED. 


NEW     YORK: 

WOOLWORTH,   AINSWORTH,    &    CO. 
1871. 


Entered  according  to  Act  of  Congress,  in  the  year  1869,  by 

W.    J.    ROLFE    AND   J.    A.    GILLET, 

In  the  Clerk's  Office  of  the  District  Court  of  the  District  of  Massachusetts. 


By  the  same  Authors, 

HANDBOOK  OF  CHEMISTRY. 
HANDBOOK  OF  THE  STARS. 

These  are  elementary  manuals  of  Chemistry  and  Astronomy,  on  the  same  plan  ai 
this  book.  In  each  of  the  three,  the  more  difficult  and  theoretical  portions  of  the  sub- 
ject are  treated  in  the  Appendix. 

Also, 

THE  CAMBRIDGE  COURSE  OF  PHYSICS, 

IN    THREE  VOLUMES : 

1.  CHEMISTRY. 
II.  NATURAL  PHILOSOPHY. 
III.  ASTRONOMY. 


CAMBRIDGE: 

PRESS  OF  JOHN  WILSON   AND  SON. 


PREFACE. 


l'7/ 


THE  great  favor  with  which  the  Natural  Philosophy 
of  the  Cambridge  Physics  has  been  received  has  en- 
couraged the  authors  to  comply  with  the  urgent  demand 
for  a  brief  and  elementary  text-book  on  the  same  subject. 
It  is  impossible  to  prepare  a  book  which  shall  be  adapted 
to  all  schools ;  but  it  is  hoped  that  the  plan  of  this  man- 
ual is  such  that  it  may  meet  the  wants  of  quite  a  wide 
range  of  schools.  In  the  body  of  the  book,  little  is  at- 
tempted beyond  a  clear  and  brief  statement  and  illus- 
tration of  those  facts  in  Physics  which  are  of  special 
importance  on  account  of  their  practical  or  theoretical 
bearing.  It  is  hoped  that  this  part  may  furnish  all  that 
is  needed'  for  the  higher  classes  in  Grammar  Schools. 
This  is  followed  by  an  Appendix,  which  contains  chap- 
ters on  the  physics  of  the  atmosphere,  on  the  theory  of 
molecular  motions,  and  on  the  origin  and  transformation 
of  energy.  This  part  is  intended  to  fit  the  book  for  the 
use  of  those  High  Schools  which  have  not  time  for  a 
larger  work.  There  is  no  overlapping  of  subjects ;  but ' 
the  Appendix  is  a  more  difficult  chapter  which  naturally 
follows. 

The  material  of  the  book  is  drawn  in  the  main  from 
the  sources  enumerated  in  the  Preface  of  the  larger 
Natural  Philosophy. 

We  have  not  forgotten  the  great  advance  recently 
made  in  the  practical  applications  of  the  physical  forces, 


IV  PREFACE. 

as  well  as  in  the  theories  of  the  science.  In  the  body  of 
the  book,  we  have  given  the  preference  to  facts  which  are 
of  practical  interest.  It  is  for  this  reason  that  frictional 
electricity  occupies  so  little  space  compared  with  current 
electricity.  A  few  years  ago,  frictional  electricity  held 
the  first  place  in  the  school  books ;  but  in  practical 
importance  it  has  dwindled  into  insignificance,  while 
voltaic  and  magnetic  electricity  have  become  of  im- 
mense value  in  the  arts.  Electricity  now  means  some- 
thing more  than  toy  experiments  with  attractions  and 
repulsions,  and  the  explosion  of  gunpowder  and  gaseous 
mixtures. 

If  the  teacher  thinks  that  we  have  varied  somewhat 
from  our  rule  in  bringing  the  double  refraction  and 
polarization  of  light  into  the  body  of  the  book,  'he  can 
omit  those  sections.  • 

The  chapter  on  Machines  is  certainly  not  too  full  for 
boys  j  and  it  is  very  easy  to  abridge  it  for  classes  of 
girls. 

The  Appendix  also  contains  problems  and  notes 
which  give  a  full  account  of  the  apparatus  needed  for 
the  book,  and  directions  for  performing  difficult  experi- 
ments. For  numerous  illustrations  which  the  teacher 
can  use  in  oral  instruction,  we  would  refer  him  to  our 
larger  Natural  Philosophy.  These  could  not  be  added 
here  without  making  the  book  too  bulky ;  and  it  is  our 
conviction  that  such  illustrations  come  from  the  teacher's 
lips  with  a  force  which  no  written  statement  can  give 
them. 

«   CAMBRIDGE,  February  15,  1869. 


TABLE  OF   CONTENTS. 


COHESION  AND  ADHESION. 

PAGE 

COHESION ' ...'...  3 

SUMMARY 16 

ADHESION 18 

SUMMARY 27 

MECHANICS. 

PRESSURE 29 

WEIGHT 29 

CENTRE  OF  GRAVITY -31 

SUMMARY 36 

PRESSURE  OF  LIQUIDS 36 

SPECIFIC  GRAVITY 43 

SUMMARY 46 

PRESSURE  OF  GASES 47 

SUMMARY  . 59 

MOTION 60 

FIRST  LAW  OF  MOTION 60 

SECOND  LAW  OF  MOTION 62 

THIRD  LAW  OF  MOTION ..65 

SUMMARY " 68 

THE  PENDULUM 69 

SUMMARY 73 

MACHINES 74 

THE  LEVER 74 


VI  CONTENTS. 

PAG* 

THE  WHEEL  AND  AXLE 78 

THE  PULLEY 82 

THE  INCLINED  PLANE 85 

THE  WEDGE 86 

THE  SCREW 87 

SUMMARY 87 

WATER  POWER 89 

STEAM  POWER  , 91 

SUMMARY  .    .     .  ' 99 

SOUND. 

NATURE  AND  PROPAGATION  OF  SOUND  .  .  .  100 

SUMMARY 104 

MUSICAL  SOUNDS 105 

SUMMARY 116 

MUSICAL  INSTRUMENTS 117 

STRINGED  INSTRUMENTS 117 

SUMMARY  . 118 

WIND  INSTRUMENTS 119 

SUMMARY 126 

SOUNDING  FLAMES 127 

SUMMARY  .  .  .  .  , 128 

THE  HUMAN  VOICE  AND  EAR 128 

SUMMARY  .  .  .  , 130 

LIGHT. 

PROPAGATION  OF  LIGHT !3i 

RADIATION,  REFLECTION,  AND  REFRACTION  ....  131 

SUMMARY 138 

DISPERSION,  ABSORPTION,  INTERFERENCE,   AND   PO- 
LARIZATION        139 

SUMMARY 146 


CONTENTS.  Vli 

PAGE 

OPTICAL  INSTRUMENTS     . 147 

LENSES 147 

SUMMARY 150 

THE  EYE 150 

SUMMARY    . 163 

THE  MICROSCOPE  AND  TELESCOPE 164 

THE  MAGIC  LANTERN 168 

SUMMARY 168 

MIRRORS * 169 

SUMMARY 172 

HEAT. 

PROPAGATION  OF  HEAT 173 

SUMMARY 176 

EFFECTS  OF  HEAT  ON  BODIES 177 

SUMMARY 188 

THERMAL  INSTRUMENTS 190 

SUMMARY 194 

ELECTRICITY. 

MAGNETISM    . 195 

SUMMARY 198 

VOLTAIC  ELECTRICITY 198 

SUMMARY 204 

ELECTRO-MAGNETISM 206 

SUMMARY 213 

ELECTROLYSIS 214 

SUMMARY 217 

POWER  OF  THE  CURRENT  TO  DEVELOP  HEAT 

AND   LIGHT 218 

SUMMARY 219 


Vlll  CONTENTS. 

PAGE 

MAGNETO-ELECTRICITY 219 

SUMMARY 221 

THERMO-ELECTRICITY 222 

SUMMARY 223 

FRICTIONAL  ELECTRICITY 223 

SUMMARY 229 

APPENDIX. 

THE  PHYSICS  OF  THE  ATMOSPHERE  ..*....  231 
MOLECULAR  MOTION  AS  MANIFESTED  IN  SOUND, 

LIGHT,  HEAT,  AND  ELECTRICITY 266 

SOURCES  AND  CONVERSION  OF  ENERGY 275 

FRENCH  WEIGHTS  AND  MEASURES 285 

PROBLEMS 287 

NOTES  ON  EXPERIMENTS ,  .  .  .  .  295 

THE  SPECTROSCOPE  AND  THE  DIFFERENT  KINDS  OF 

SPECTRA 305 

PHOTOGRAPHY 308 

NOTES  .  .' 310 

QUESTIONS  FOR  REVIEW  AND  EXAMINATION  .  .  .  315 


INDEX 


325 


HANDBOOK 


OF 


NATURAL    PHILOSOPHY. 


COHESION. 


1 .  Matter  is  made  up  of  Molecules.  —  When  a  piece 
of  ice  is  heated  to  a  temperature  of  32°,  it  melts  and  be- 
comes water.      The  particles  of  the  ice  hold   together 
firmly,  but  in  melting  they  have  been  loosened  so  that 
they  move  among  themselves  with  the  greatest  ease. 

If  water  be  heated  to  a  temperature  of  212°,  it  boils 
and  becomes  steam.  Its  particles  are  still  farther  sepa- 
rated from  one  another. 

The  particles  of  which  all  bodies  are  built  up,  and 
which  are  loosened  and  separated  when  a  solid  melts 
and  a  liquid  boils,  are  called  molecules.  Molecule  is  a 
word  from  the  Latin,  and  means  a  little  mass. 

2.  Molecules  are  exceedingly  small.  —  It  is  impossible 
to  grind  a  solid  so  fine  as  to  convert  it  into  a  liquid.     A 
piece  of  gold  may  be  divided  into  particles  so  small,  that 
each  can  barely  be  made  out  with  a  powerful  microscope, 
yet  the  gold  is  solid  still.     When  heated,  however,  the 
pulverized  gold  becomes  a  liquid ;   that  is,  each  minute 
piece    is    separated   into   molecules.      These    molecules, 
then,  are  much  too  small  to  be  seen  with  the  best  micro- 
scope. 

3.  The  Molecules  are  not  in  actual  con-         Fis-  «• 
tact.  —  If  a  brass  ball,  which   at  the  ordi- 
nary temperature  will  just  pass  through  a 

ring  (Figure  i),  be  plunged  into  a  freezing 
mixture  and  left  until  it  becomes  very  cold, 
it  will  then  pass  through  the  ring  very  eas- 
ily,  not  touching  it  at  all. 


COHESION. 


Fig.  2. 


If  a  bulb  with  a  projecting  tube  be 
filled  with  water  up  to  a  certain  point 
on  the  tube,  and  the  bulb  be  then 
plunged  into  a  freezing  mixture 
(Figure  2),  the  water  will  fall  in 
the  tube. 

If  a  similar  bulb  be  filled  with  air, 
and  the  end  of  the  tube  be  held  under 
water  (Figure  3),  and  the  bulb  be 
cooled  by  means  of  a  freezing  mix- 
ture, the  water  at  once  rises  in  the  tube ;  showing  that 
the  air  occupies  less  space  when  cooled. 

Fig.  3.  We  find,  then,  that  solids,  liquids, 

and  gases  contract  when  cooled  ;  and 
there  seems  to  be  no  limit  to  this 
contraction,  for  they  continue  to  con- 
tract, however  much  they  are  cooled. 
Now  this  contraction  is  best  ex- 
plained by  supposing  that  the  mole- 
cules come  nearer  together ;  and 
since,  so  far  as  we  know,  a  body  may 
continue  to  contract  indefinitely,  it  follows  that  the  mole- 
cules are  never  in  actual  contact. 

4.  The  Spaces  between  the  Molecules  are  immense 
in  comparison  'with  the  Size  of  the  Molecules. — 
Though  the  spaces  between  the  molecules  are  very 
minute,  since  they  cannot  be  discerned  even  with  the 
most  powerful  microscope,  there  are  good  reasons  for 
believing  that  they  are  immense  in  comparison  with  the 
bulk  of  the  molecules  themselves.* 


*  The  molecules  of  a  body  maybe  compared  to  the  earth,  sun, 
moon,  and  stars,  and  the  spaces  between  the  molecules  to  the 
spaces  between  these  heavenly  bodies.  If  we  imagine  a  being 
small  enough  to  live  on  one  of  the  molecules  in  the  centre  of  a 
stone,  as  we  live  on  the  earth,  he  would,  on  looking  out  into  the 


COHESION.  5 

5.  An  Attractive  Force  and  a  Reptdsive  Force  act 
between  Molecules.  —  If  we  attempt   to   pull  any  solid 
asunder,    we    perceive    at    once    that    the    particles    of 
which  it  is  composed  are  held  together  more   or   less 
firmly.      That  which  holds  them  together  is  called  an 
attractive  force.     If  a  glass  rod  be  dipped  into  water,  a 
drop  hangs  from  its  end  when  taken  out.     This  drop  is 
made  up  of  molecules  which  are  evidently  held  together. 
In  the  case  of  liquids,  the  molecules  are  held  together 
but  feebly,  and  the  attractive  force  seems  to  be  slight. 

If  a  rubber  bag  partially  filled  with  air,  and  closed  so 
as  to  be  air-tight,  be  placed  under  the  receiver  of  an  air- 
pump,  and  the  air  exhausted  from  the  receiver,  the  air 
within  the  bag  will  at  once  expand,  as  we  see  by  the 
filling  out  of  the  bag.  This  shows  that  gases  when  left 
to  themselves  expand  ;  that  is,  their  molecules  separate. 

The  force  which  separates  the  molecules  is  called  a 
repulsive  force. 

Since  these  forces  act  between  molecules,  they  are 
called  molecular  forces. 

6.  These  two  Forces  act  together.  —  A  brass  ball  (see 
Figure  i )  which  will  just  pass  through  a  ring  at  the  ordi- 
nary temperature,  will  not  pass  through  the  ring  after 
being  heated ;    showing  that  the  molecules  of  the  ball 
have  been  pushed  apart.     If,  however,  while  the  solid  is 
heating,  we  attempt  to  pull  it  asunder,  it  resists ;  show- 
ing that  the  molecules  are  still  held  together  by  an  attrac- 
tive  force.      It   is  evident,   then,   that  both    forces    act 
together. 

space  about  him,  see  here  and  there,  at  immense  distances,  other 
molecules,  as  we  see  the  scattered  stars  in  the  heavens  at  night. 
The  molecules,  though  exceedingly  minute,  are  perfectly  distinct 
and  definite  masses,  like  the  earth,  moon,  and  stars ;  and  they 
are  separated  by  spaces  many  thousand  times  as  great  as  that 
occupied  by  each  molecule. 


6  COHESION. 

7.  The  Three  States  of  Matter.  —  When  the  attrac- 
tive force   is   considerably  stronger   than   the   repulsive 
force,  matter  is  in  the  solid  state.;  when  the  two  forces 
are  nearly  balanced,  it  is  in  the  liquid  state;  and  when 
the  repulsive  force  is  the  stronger,  in  the  gaseous  state. 

8.  Cohesion  and  Adhesion.  —  The  force  which  holds 
the  molecules  of  a  solid  or  liquid  together  is  evidently  the 
excess  of  the  attractive  over  the  repulsive  force  ;  for  if  the 
two  forces  were  just  equal,  they  would  just  neutralize 
each  other,  and  the  molecules  would  not  be  held  together 
in  the  least. 

In  the  case  of  iron  or  water,  molecules  of  the  same 
kind  are  held  together.  When  we  mark  on  a  blackboard 
\vith  a  piece  of  chalk,  or  write  on  paper  with  ink,  mole- 
cules of  different  kinds  are  held  together. 

The  force  which  holds  together  molecules  of  the 
same  kind  is  called  cohesion ;  that  which  holds  together 
molecules  of  different  kinds^is  called  adhesion. 

9.  These  Forces   act   only  through   insensible    Dis- 
tances. —  Two  pieces  of  lead  will  not  cohere  if  their  sur- 
faces are  rough ;  but  if  we  make  them  perfectly  smooth 
and  clean,  and  press  them  firmly  together,  they  cohere 
quite  strongly.     Plates  of  glass,  from  simply  resting  upon 
one  another  in  the  warehouse,  have  been  known  to  cohere 
so  firmly  that  they  would  break  elsewhere  as  readily  as 
where  they  came  in  contact. 

10.  Solids  have  considerable  Cohesion.  —  Matter,  as 
we  have  seen,  exists  in  three  states,  the  solid,  the  liquid, 
and  the  gaseous.     The   distinguishing  characteristic  of 
solids  is  that  the  attractive  considerably  exceeds  the  re- 
pulsive force.      In  solids,  therefore,  the  cohesion  is  al- 
ways  considerable.      The   various    properties   of  solids 
result  from  modifications  of  this  molecular  force. 

1 1 .  Tenacity.  —  We  find  on  trial  that  it  is  much  eas- 
ier to  pull  in  two  a  rod  of  lead  than  a  rod  of  steel  of  the 


COHESION.  7 

same  thickness;  showing  that  the  molecules  of  some 
solids  cohere  more  strongly  than  those  of  others. 

When  a  solid  is  thus  pulled  in  two,  it  is  said  to  be  rup- 
tured. The  power  which  a  solid  has  of  resisting  rup- 
ture is  called  tenacity. 

The  relative  tenacity  of  different  solids  is  determined 
by  finding  how  much  force  is  required  to  rupture  rods  of 
the  same  thickness  made  of  them.  If  it  takes  twice  as 
much  force  to  pull  asunder  one  rod  as  another,  the  first 
solid  is  said  to  have  twice  the  tenacity  of  the  second: 

The  relative  tenacity  of  solids  may  be  found  by  means 
of  a  machine  called  a  dynamometer.  This  name  is  made 
up  of  two  Greek  words,  and  means  force-measurer. 

Fig.  4- 


One  form  of  the  machine  is  shown  in.  Figure  4.  It 
consists  of  a  heavy  iron  frame,  at  one  end  of  which  is  a 
box  containing  a  stout  steel  spring.  A  pointer  connected 
with  this  spring  moves  over  a  graduated  arc  on  the  top 
of  the  box.  On  the  frame  are  two  movable  blocks,  or 
slides,  one  of  which  is  attached  to  the  spring,  while  the 
other  may  be  carried  backward  and  forward  by  a  screw 
and  crank. 

The  rod  whose  tenacity  is  to  be  tried  is  stretched  be- 
tween the  two  slides,  and  the  crank  is  then  slowly  turned 
so  as  to  pull  upon  the  rod  until  it  breaks.  The  force  thus 
brought  to  bear  upon  the  rod  bends  the  spring ;  and  the 


8  COHESION. 

position  of  the  pointer  when  the  rod  breaks  shows  how 
much  force  it  took  to  break  it. 

12.  Hardness  and  Softness.  —  If  we  indent  apiece  of 
india-rubber  with  the  finger-nail,  or  strike  a  piece  of  lead 
a  smart  blow  with  a  hammer,  we  see  that  it  is  possible 
to  displace  the  molecules  of  a  solid.      When  it  is  easy  to 
displace  the  molecules,  as  in  the  case  of  wax,  the  solid  is 
called  soft ;  when  it  is  difficult  to  displace  them,  as  in 
the  case  of  glass,  the  solid  is  called  hard. 

To  find  which  of  two  solids  is  the  harder,  see  which 
will  scratch  the  other.  The  one  which  scratches  is  al- 
ways harder  than  the  one  scratched.  Diamond  is  the 
hardest  solid  known ;  hence  it  is  used  for  cutting  glass, 
which  is  also  a  very  hard  substance. 

13.  Elasticity,  Brittleness,   Ductility,  and  Mallea- 
bility.—  When  molecules  have  been  displaced,  one  of 
three  results  must  follow,  —  they  will  return  to  their  orig- 
inal positions  as  soon  as  they  are  left  to  themselves,  or 
they  will  take  up  new  positions,  or  they  will  fall  entirely 
asunder. 

If  we  bend  a  steel  rod  moderately,  it  straightens  as 
soon  as  it  is  released ;  showing  that  displaced  molecules 
sometimes  tend  to  return  to  their  former  positions.  This 
tendency  of  molecules  to  return  to  their  original  posi- 
tions after  being  displaced  is  called  elasticity. 

A  steel  rod  may  be  bent  a  good  deal,  and  yet  straighten 
when  released ;  but  if  it  be  bent  beyond  a  certain  point, 
it  will  no  longer  straighten,  showing  that  the  molecules, 
after  they  have  been  displaced  beyond  a  certain  limit, 
tend  to  remain  in  their  new  positions.  The  greatest  ex- 
tent to  which  the  molecules  of  a  solid  can  be  displaced, 
and  yet  go  back  to  their  former  positions,  is  called  the 
limit  of  elasticity  for  that  solid.  All  solids  are  found  to 
be  elastic,  but  they  differ  very  much  in  the  limit  of  their 
elasticity.  The  molecules  of  steel  and  india-rubber  can 


COHESION.  9 

be  displaced  a  good  deal  without  becoming  fixed  in  new 
positions,  while  those  of  glass  ^and  pipe-clay  can  be  dis- 
placed but  slightly. 

If  a  glass  rod  be  bent  within  a  certain  limit,  it  will 
straighten  when  released  ;  but  if  it  be  bent  beyond  this 
limit,  it  will  not  remain  bent,  but  w^ill  break.  When  the 
displaced  molecules  cannot  take  up  permanently  new 
positions,  the  solid  is  said  to  be  brittle.  Hard  solids  are 
likely  to  be  brittle  also  ;  but  hardness  and  brittleness  are, 
as  we  have  seen,  entirely  different  things. 

When  the  molecules  after  being  displaced  can  take 
up  permanently  new  positions,  the  solid  is  ordinarily 
described  as  malleable  or  ductile.  It  is  said  to  be  mal- 
leable when  it  can  be  hammered  or  rolled  out  into 
sheets;  ductile  when  it  can  be  drawn  out  into  wire. 

Gold  is  one  of  the  most  malleable  of  the  metals.  In 
the  manufacture  of  gold  leaf,  it  is  hammered  out  into 
sheets  so  thin  that  it  takes  from  300,000  to  35-0,000  of 
them  to  make  the  thickness  of  a  single  inch. 

Wire  is  made  by  drawing  a  rod  of  metal  through  a 
series  of  conical  holes  in  a  hardened  steel  plate.  Each 
hole  is  a  little  smaller  than  the  preceding,  so  that  the  rod 
becomes  lengthened  and  diminished  in  thickness  as  it  is 
drawn  through  one  after  another. 

In  the  drawing  of  iron  wire,  the  molecules  are  sepa- 
rated, yet  the  tenacity  of  the  iron  is  greatly  increased,  so 
that  fine  iron  wire  is  the  most  tenacious  of  substances. 
A  bar  one  inch  square  of  the  best  wrought-iron  will  sus- 
tain a  weight  of  thirty  tons ;  a  bundle  of  wires  one-tenth 
of  an  inch  in  diameter,  containing  the  same  quantity  of 
material,  will  sustain  a  weight  of  from  thirty-six  to  forty 
tons ;  and  if  the  wires  have  a  diameter  of  only  one-twen- 
tieth or  one-thirtieth  of  an  inch,  the  same  quantity  will 
sustain  from  sixty  to  ninety  tons.  Hence  cables  made  of 
fine  iron  wire  twisted  together  are  much  stronger  than 


10 


COHESION. 


bars  or  chains  of  the  same  weight.     The  cables  of  sus- 
pension bridges  are  madejn  this  way. 

The  following  Table  gives  the  most  useful  metals  in 
the  order  of  their  tenacity,  malleability  (both  under  the 
hammer  and  the  rolling-mill),  and  ductility:  — 


Tenacity. 

Iron 

Copper 

Platinum 

Silver 

Zinc 

Gold 

Lead 

Tin 


14.  Solids  are  somewhat  Compressible.  —  Some 
metals  are  permanently  diminished  in  bulk  by  hammer- 
ing ;  and  so  also  by  the  pressure  to  which  they  are  sub- 
jected in  the  process  of  coining.  The  stone  columns  of 
buildings  are  frequently  shortened  by  the  great  weight 
resting  upon  them.  This  was  found  to  be  the  case  with 
the  columns  supporting  the  dome  _of  the  Pantheon  at 


^ability  under 
;  Hammer. 

Malleability  under 
the  Rolling-Mill. 

Ductility. 

Lead 

Gold 

Platinum 

Tin 

Silver 

Silver 

Gold 
Zinc 
Silver 

Copper 
Tin 
Lead 

Iron 
Copper 
Gold 

Copper 
Platinum 

Zinc 
Platinum 

Zinc 
Tin 

Iron 

Iron 

Lead 

15.  The  Molecules  sometimes  arrange  themselves  in 
Crystals.  —  If  alum  be  added  to  hot  water  as  long  as  it 
will  dissolve,  and  then  the  water  be  allowed  to  cool 
slowly,  a  part  of  the  alum  will  be  deposited  on  the  bot- 
tom of  the  dish,  not  in  a  confused  mass,  but  in  beautiful 
and  symmetrical  forms,  -called  crystals. 

Melt  some  sulphur  in  a  crucible,  and  let  it  cool  slowly 
till  a  crust  forms  on  the  surface  ;  then  carefully  break 
the  crust,  and  pour  off  the  remaining  liquid.  The 
crucible  (Figure  5)  will  be  found  lined  with  delicate 
needle-shaped  crystals. 


COHESION.  II 

As  a  general  rule,  the  molecules 
of  a  solid  tend  to  arrange  themselves 
in  crystals.  The  same  solid  usually 
crystallizes  in  the  same  form,  but 
different  solids  in  different  forms. 

In  the  cases  of  crystallization 
which  we  have  already  described, 
the  solid  is  first  brought  to  the 
liquid  state,  that  the  molecules  may 
have  freedom  of  motion.  The  building  of  a  crystal  out 
of  molecules  is  much  like  building  a  house  out  of  bricks. 
The  bricks  must  be  taken  one  by  one  and  laid  in  regular 
order  before  they  are  cemented  together.  So,  in  forming 
a  crystal,  the  molecules  must  be  arranged  one  by  one  in 
regular  order  before  they  are  fastened  together  by  the 
cohesive  force. 

Large  crystals  of  many  solids  can  be  obtained  by  dis- 
solving as  much  of  the  solid  as  is  possible  in  cold  water, 
and  then  setting  it  away  in  a  shallow  dish  where  it  will  be 
free  from  dust  and  disturbance,  and  allowing  the  water 
to  evaporate  very  slowly.  The  more  gradual  the  for- 
mation, the  larger  are  the  crystals.  The  large  crystals 
seen  in  cabinets  of  minerals  were  probably  centuries  in 
forming.  The  water  in  which  the  solid  was  dissolved 
found  its  way  into  a  cavity  of  a  rock,  and  there  slowly 
evaporated. 

The  tendency  of  the  molecules  to  form  crystals  is  strik- 
ingly_  shown  in  cannon  which  have  been  many  times 
fired,  and  in  shafts  of  machinery  and  axles  of  car-wheels 
which  are  continually  jarred.  Such  bodies  often  become 
brittle,  and  on  breaking  show  the  smooth  faces  of  the 
crystals  which  have  been  formed.  The  continued  jar- 
ring gives  the  molecules  a  slight  freedom  of  motion, 
and  crystals  are  slowly  built  up;^" 

Many  solids  are  crystalline  in  structure  which  do  not 


12  COHESION. 

appear  to  be  so.  Thus  a  piece  of  ice  is  a  mass  of  the 
most  perfect  crystals,  but  they  are  so  closely  packed  to- 
gether that  we  cannot  readily  distinguish  them. 

1 6.  The  Molecules  cohere  more  strongly  on  some  sides 
than  on  others.  —  It  is  easy  to  cleave  a  piece  of  mica  or 
other  crystal  in  certain  directions,  but  difficult  to  cleave 
it   in   other   directions.      The    molecules    cohere    more 
strongly  on  some  sides  than  on  others.     Iron  and  other 
solids  are  not  so  tenacious  when  crystalline  in  structure 
as  when  not  crystalline.     This  is  because  the  molecules 
in  crystals  are  arranged  in  layers,  so  that  the  weakest 
sides  are  brought  face  to  face. 

17.  Annealing  and  Tempering.  —  If  melted  glass  be 
dropped  into  cold  water,  it  forms  the  well-known  Ru- 
pert's drops,  which  are  so  brittle  that,  if  we  break  off  the 
small  end  or  scratch  them  slightly  w.ith  a  file,  they  fly  in 
pieces.     When   glass  is   allowed   to    cool  in  the    air   at 
the  ordinary  temperature,  it  is  also  very  brittle.    In  order 
to   make  it  tough  enough  for  ordinary  use,  it  must  be 
cooled  very  slowly,  or  annealed.     This  is  done  by  pass- 
ing it  slowly  through  a  long  oven,  which  is  kept  very  hot 
at  one  end  and  cool  at  the  other. 

Steel,  also,  when  suddenly  cooled  from  a  high  temper- 
ature, is  very  hard  and  brittle  ;  but  when  slowly  cooled, 
it  is  very  tough  and  pliable.  The  process  of  giving  steel 
various  degrees  of  hardness  is  called  tempering.  The 
steel  is  first  heated  white  hot,  and  then  suddenly  cooled 
by  plunging  it  into  cold  water.  It  is  thus  rendered  very 
brittle.  It  is  then  reheated,  and  allowed  to  cool  slowly. 
When  it  is  to  be  made  quite  hard,  it  is  reheated  but 
slightly ;  when  quite  soft,  it  is  reheated  a  good  deal. 
The  more  it  is  reheated,  the  softer  it  becomes  on  cooling. 
These  different  conditions  of  glass  and  steel  are  probably 
owing  to  differences  in  the  arrangement  of  the  molecules. 

1 8.  Liquids  have  little  Cohesion.  —  The  chief  char- 


COHESION.  13 

acteristic  of  liquids  is  that  the  attractive  and  repulsive 
forces  acting  between  the  molecules  are  very  nearly  bal- 
anced, the  attractive  force  being  slightly  the  greater. 
Hence  in  liquids  the  cohesion  is  slight,  and  the  mole- 
cules are  free  to  move  among  themselves. 

If  a  piece  of  lead  be  carefully  measured,  then  melted 
and  measured  again,  it  will  be  found  to  have  increased 
in  bulk.  Hence,  when  any  substance  is  in  a  liquid 
state,  the  molecules  are  farther  apart  than  when  it  is 
in  a  solid  state.  This  explains  why,  in  moulding  bul- 
lets, the  mould  is  never  quite  filled  by  the  bullet. 

There  are  a  few  exceptions  to  this  rule.  If,  for  in- 
stance, a  bottle  be  filled  with  water  and  tightly  corked,  and 
allowed  to  freeze,  the  bottle  will  burst.  The  molecules 
of  ice,  then,  must  be  farther  apart  than  those  of  water. 

19.  Liqtiids  are  but  slightly  Com-  Fis-  6- 

pressible.  —  The  apparatus  repre- 
sented in  Figure  6  consists  of  a  very 
thick  vessel  of  glass  closed  at  top 
and  bottom.  Within  the  vessel  are  a 
piston  which  can  be  moved  by  the 
thumb-screw  at  the  top,  and  a  glass 
bulb  which  is  prolonged  by  a  very 
fine  tube,  bent  as  represented.  Fill 
the  bulb  and  tube  with  any  liquid, 
as  water,  and  plunge  the  end  of  the 
tube  in  the  mercury  which  covers 
the  bottom  of  the  vessel.  Then  fill 
the  vessel  with  water,  and  apply 
pressure  by  turning  the  screw.  The 
mercury  will  rise  in  the  tube,  show- 
ing that  the  liquid  in  the  bulb  has 
been  compressed.  This  compres- 
sion, however,  is  but  slight,  amount- 
ing at  most  to  a  few  millionths  of  the  bulk  of  the  liquid. 


14  COHESION. 

20.  Liquids  are  perfectly  Elastic.  —  However  much 
the  screw,  in  the  above  experiment,  may  be  turned  down, 
or  however  long  it  may  be   left,  the  moment  we  loosen 
it  the  mercury  will  fall  inside  the  tube  to  a  level  with 
the  mercury  outside ;  showing  that  liquids  are  perfectly 
elastic. 

This  elasticity  is  developed  only  when  the  liquid  is 
compressed;  that  is,  when  the  molecules  have  been 
brought  nearer  together.  In  whatever  other  way  the 
molecules  may  be  displaced,  they  show  no  tendency  to 
return  to  their  former  positions. 

21.  The  Molecules  in  Liquids  when  left  to  themselves 
collect  into  Spheres.  —  If  a  mixture  of  water  and  alcohol 
be  made,  which  is  just  as  heavy  as  sweet-oil,  bulk  for 
bulk,  and  a  quantity  of  the  oil  be  carefully  introduced 
into  the  centre  of  this  mixture  by  means  of  a  dropping- 
tube,  the  oil  will  neither  rise  nor  sink,  but  gather  into  a 
beautiful  sphere.     This  shows  that  when  the  molecules 
of  a  liquid  are  left  to  themselves,  they  at  once  collect 
into  spheres. 

Rain-drops,  dew-drops,  and  the  manufacture  of  shot 
illustrate  this  tendency  of  the  molecules  of  liquids.  In 
making  shot,  melted  lead  is  poured  through  a  sieve  at 
the  top  of  a  very  high  tower,  and  the  drops  in  falling 
take  the  form  of  spheres,  which  become  solid  before  they 
reach  the  bottom. 

22.  Gases  have  no  Cohesion. — In  gases,  as  has  al- 
ready been  shown,  the  repulsive  molecular  force  exceeds 
the  attractive.  Hence  there  is  no  cohesion  in  this  state 
of  matter,  and  the  molecules  move  among  themselves 
with  greater  freedom  than  those  of  liquids. 

The  molecules  of  any  substance  are  farther  apart  in  the 
gaseous  state  than  in  either  the  solid  or  the  liquid  state. 
Fill  a  test-tube  nearly  full  of  water,  then  close  it  tightly 
with  a  cork  through  which  a  fine  tube  passes  nearly  to 


COHESION. 


the  bottom  of  the  test- 
tube.  Boil  the  water  so 
as  to  convert  a  portion  of 
it  into  steam,  which  is  a 
gas,  and  the  water  will 
be  driven  forcibly  out  of 
the  fine  tube  (Figure  7)  ; 
showing  that  the  steam 
occupies  more  space  than 
the  water  from  which  it 


Fig.  7. 


comes. 


23 


are    readily 


Compressible.  —  The  Fig- 
ure represents  a  U-tube 
closed  at  one  end  and 
open  at  the  other,  with  a 
nipper-tap  *  at  the  bend. 
Pour  in  mercury  enough 
to  cover  the  bend.  The 
closed  end  is  now  filled 
with  air.  Pour  in  more 
mercury  and  this  column 
of  air  rapidly  shortens; 
showing  that  gases  are 
highly  compressible. 
24.  Gases  are  perfectly  Elastic.  —  Open  the  nipper- 
tap  that  the  mercury  may  run  out,  and  it  is  entirely 
driven  out  of  the  closed  arm  of  the  tube.  To  prove  that 
it  is  the  elasticity  of  the  air  which  drives  out  the  mercury 
from  this  arm,  fill  the  closed  arm  and  a  part  of  the  open 
arm  with  mercury,  and  open  the  nipper- tap.  The  mer- 
cury will  flow  out  from  the  open  arm,  and  not  from  the 
closed  arm. 

Air  and  other  gases  are  perfectly  elastic. 


*  See,  in  Appendix,  Notes  on  Experiments. 


i6 


COHESION. 


SUMMARY. 


Matter  is  made  up  of  particles  too  small  to  be  seen, 
called  molecules,  (i,  2.) 

These  molecules  are  not  in  actual  contact  with  one 
another.  It  is  probable  that  the  spaces  which  separate 
them  are  immense  in  comparison  with  the  size  of  the 
molecules  themselves.  (3,  4.) 

There  is  an  attractive  molecular  force,  which  holds 
the  molecules  together,  and  a  repulsive  molecular  force, 
which  pushes  the  molecules  apart.  (5.) 

These  two  molecular  forces  act  together,  and  the  re- 
pulsive molecular  force  is  increased  by  heat.  (6.) 

There  are  three  states  of  matter :  the  solid  state,  in 
which  the  attractive  force  is  considerably  the  greater ; 
the  liquid  state,  in  which  the  two  forces  are  nearly  equal ; 
and  the  gaseous  state,  in  which  the  repulsive  force  is  the 
greater.  (7.) 

The  force  which  holds  together  molecules  of  the  same 
kind  is  called  Cohesion ;  that  which  holds  together  mole- 
cules of  different  kinds,  Adhesion.  (8.) 

Cohesion  is  the  excess  of  the  attractive  over  the  repul- 
sive molecular  force.  In  solids,  it  is  comparatively  strong ; 
in  liquids,  it  is  weak  ;  in  gases,  it  does  not  exist. 

The  properties  of  solids  depend  on  the  action  of  the 
cohesive  force.  (10.) 

The  tenacity  of  a  solid  is  its  power  of  resisting  rup- 
ture, (n.) 

A  solid  is  called  hard  when  it  is  difficult  to  displace 
its  molecules ;  soft,  when  it  is  easy  to  displace  them. 

(12.) 

Elasticity  is  the  tendency  of  the  molecules,  on  being 
displaced,  to  return  to  their  original  positions.  All  solids 
are  elastic,  but  differ  greatly  in  the  limit  of  their  elas- 
ticity. 


COHESION.  17 

A  solid  is  said  to  be  brittle  when  its  molecules  cannot 
take  up  permanently  new  positions. 

It  is  said  to  be  malleable  or  ductile  when  they  can  take 
permanently  new  positions  :  malleable,  -when  it  can  be 
hammered  or  rolled  into  sheets ;  ductile,  when  it  can  be 
drawn  into  wire.  (13.) 

Solids  are  somewhat  compressible.     (14.) 

The  cohesive  force  often  arranges  the  molecules  of  a 
solid  into  regular  forms  called  crystals.  (15.) 

The  cohesive  force  is  stronger  on  some  sides  of  the 
molecule  than  on  others.  (16.) 

The  molecules  are  farther  apart  in  the  liquid  than  in 
the  solid  state  ;  yet  liquids  are  less  compressible  than 
solids.  (19.) 

Liquids  are  perfectly  elastic  j  but  their  elasticity  is 
developed  only  when  the  molecules  are  brought  nearer 
together.  (20.) 

The  molecules  of  a  liquid,  when  acted  upon  only  by 
cohesion,  tend  to  collect  into  spheres.  (21.) 

In  the  gaseous  state,  the  molecules  are  farther  apart 
than  in  the  liquid  state.  (22.) 

Gases  are  readily  compressible,  and  when  compressed 
are  perfectly  elastic.  (23,  24.) 


ADHESION. 


25.  Adhesion  between  Solids  and  Solids.  —  Adhesion 
has  already  been  defined  as  the  force  which  holds  to- 
gether unlike  molecules. 

The  sticking  of  the  chalk  to  the  blackboard,  of  the 
graphite  of  the  pencil  to  paper,  and  of  dust  to  furniture, 
prove  the  existence  of  this  force  between  solids  and 
solids.  The  use  of  the  various  cements  also  illustrates 
this  force. 

When  solids  are  held  together  by  cements,  cohesion 
and  adhesion  are  both  brought  into  play.  When,  for 
instance,  two  pieces  of  wood  are  held  together  by  means 
of  gkie,  the  adhesive  force  holds  the  wood  on  each  side 
to  the  glue,  and  cohesion  holds  together  the  molecules  of 
the  glue. 

When  furniture  breaks,  we 'often  see  that  the  wood 
splits  instead  of  separating  from  the  glue.  So  also  stones 
are  sometimes  cemented  together  so  firmly  that  the  stone 
itself  will  break  sooner  than  separate  from  the  cement. 
The  adhesive  force  between  two  solids  is  frequently 
stronger  than  the  cohesive  force  of  the  solids  them- 
selves. 

26.  Adhesion  between  Solids  and  Liquids.  —  If  we 
dip  the  hand  in  water,  it  comes  out  wet.     This  fact,  with 
others   equally    familiar,    proves   that  there    is   also    an 
adhesive  force  between  liquids  and  solids. 

27.  The  Adhesion  between  a  Liquid  and  a   Solid  is 
sometimes  not  strong  enough  to  overcome  the  Cohesion 
of  the  Liquid.  —  If  a  glass  disk  be  suspended  from  one 
pan  of  a  balance  and  counterpoised  by  weights,  and  then 


ADHESION. 


brought  in  contact  with  mercury,  it  will  require  addi- 
tional weight  to  raise  the  disk  from  the  mercury,  and  the 
disk  comes  off  dry.  This  proves,  first,  that  there  is  adhe- 


Fig.  9. 


sion  between  glass  and  mercury,  and,  secondly,  that  this 
adhesion  is  not  strong  enough  to  overcome  the  cohesion 
of  the  mercury. 

28.  The  Adhesion  between  a  Solid  and  a  Liquid  is 
sometimes  strong  enough  to  overcome  the  Cohesion  of 
the  Liquid.  —  If  a  glass  plate  be  laid  upon  the  surface 
of  water  and  then  removed,  it  comes  off  wet,  that  te  to 
say,  covered  with  a  film  of  water ;  showing  that  the  ad- 
hesion between  a  solid  and  liquid  is  sometimes  strong 
enough  to  overcome  the  cohesion  of  the  liquid. 

Since  adhesion  takes  place  only  at  the  surface,  it  is 
evident  that  we  may  increase  the  adhesion  of  a  solid 
for  a  liquid  by  increasing  the  surface  of  the  solid. 

If  we  take  a  stone,  and  break  it  in  two,  it  evidently 
has  all  the  surface  it  had  before  it  was  broken,  and,  in 
addition,  the  two  surfaces  exposed  by  the  breaking.  The 
more  it  is  broken  up,  the  more  surface  it  expos.es.  The 
readiest  way,  then,  to  increase  the  surface  of  a  solid  is  to 
pulverize  it. 


2O  ADHESION. 

If  pulverized  bone-black  be  mixed  with  vinegar,  or 
with  wine,  and  the  liquid  be  separated  again  by  pouring 
the  mixture  upon  unsized  paper  placed  inside  a  funnel, 
the  liquid  that  runs  through  will  be  colorless.  All  vege- 
table colors  can  be  removed  from  liquids  in  the  same 
way.  The  process  is  called  clarifying  the  liquid. 

Bone-black  is  obtained  by  burning  bones  in  closed  ves- 
sels. It  is  pulverized  that  it  may  present  more  surface. 
Other  substances  may  be  used  for  clarifying  liquids. 
Next  to  bone-black,  ordinary  charcoal  is  the  best  and  the. 
most  frequently  used.  The  bone-black  evidently  removes 
the  coloring  matter  by  means  of  the  adhesive  force  which 
exists  between  the  two.  The  coloring  matter  adheres  to 
the  bone-black  more  strongly  than  to  the  liquid,  or  the 
two  would  not  be  separated. 

29.  The  Adhesion  between  a  Solid  and  a  Liquid  is 
sometimes  strong  enough  to  overcome  the  Cohesion  of 
the  Solid.  —  If  some  Epsom  salts  be  put  into  water,  the 
salts  will  speedily  become  liquid.    The  adhesive  force  be- 
tween the  water  and  the  salts  overcomes  the  cohesive  force 
of  the  solid,  since  it  reduces  the  solid  to  the  liquid  state. 

30.  Three    Cases  of  Adhesion   between   Solids   and 
Licfuids. — We  have,  then,  three  well-marked  cases  of 
adhesion  between  solids  and  liquids :  — 

ist,  When  the  adhesive  force  is  not  strong  enough  to 
overcome  the  cohesion  of  the  liquid.  In  this  case,  the 
liquid  cannot  wet  the  solid. 

2d,  When  the  adhesive  force  is  strong  enough  to  over- 
come the  cohesion  of  the  liquid.  In  this  case,  the  liquid 
can  wet  the  solid. 

3d,  When  the  adhesive  force  is  strong  enough  to  over- 
come the  cohesion  of  the  solid.  In  this  case,  the  liquid 
can  dissolve  the  solid.  The  liquid  which  dissolves  the 
solid  is  called  a  solvent,  and  the  liquid  in  which  the 
solid  has  been  dissolved  is  called  a  solution. 


ADHESION.  21 

31.  Heat  promotes  Solution. — We  find  on  trial  that 
Epsom  salts  will  dissolve  more  rapidly  and  in  greater 
quantity  in  hot  than  in  cold  water.     We  have  already 
found  (i,  6)  that  heat  tends  to  overcome  cohesive  force. 
As  a  general  rule,  solids  dissolve  in  greater  quantities  and 
more  readily  in  hot  than  in  cold  liquids ;   but  there  are 
exceptions,  among  which  are  lime  and  Glauber's  salts. 

32.  Capillarity.  —  If  one  end  of  a  fine  and  clean  glass 
tube  be  put  into  water,  the  water  will  rise  inside  the  tube 
above  the  surface  of  the  water  outside.     If  one  end  of 
such  a  tube  be  put  into  mercury,  the  mercury  will  fall 
inside  the  tube  below  the  surface  of  the  mercury  outside. 
This  action  of  liquids  inside  tubes  is  called  capillarity. 
The  force  which  draws  some  liquids  into  tubes  and  pushes 
others  out,  has  been  called  capillary  force.     This  name 
is  a  convenient  one,  though  capillarity  really  results  from 
the  combined  action  of  certain  other  forces. 

We  have  seen  that  water  will  wet  glass  (28),  while 
mercury  will  not  (27).  We  have,  then,  two  distinct 
cases  of  capillarity,  corresponding  to  two  cases  of  adhe- 
sion between  solids  and  liquids  ;  for  those  liquids  'which 
'will  wet  a  tube  are  drawn  into  Fig.  10. 

it)  while  those  which  will  not  wet 
it  are  driven  out.  Mercury  will 
wet  zinc,  and  it  is  drawn  into  a 
tube  of  zinc,  just  as  water  is  into 
a  tube  of  glass. 

We  find  by  using  glass  tubes  of 
different  sizes,  that  the  finer  the  tube,  the  higher  the  water 
rises  and  the  lower  the  mercury  falls ;  that  is,  the  more 
marked  is  the  capillarity.  The  word  capillary  comes 
from  a  Latin  word  (capillaris) ,  which  t  means  hair- 
like.  The  force  was  called  capillary  because  its  action 
is  most  powerful  in  hair-like  tubes.  This  force,  however, 
acts  in  tubes  of  every  size,  and  in  fact  a  tube  is  not  ne- 


22  ADHESION. 

cessary  for  its  action.      Put  two  plates  of  glass  together 
as  represented  in  Figure  1 1 ,  and  then  dip  them  into  water 
Fig.  ii.  or  mercury.     The  water  will 

rise  between   the  plates,  and 
the  mercury  will  fall. 

33.  Illustrations  of  Capil- 
larity.—  A  lamp-wick  is  full 
of  tubes  and  pores  ;  and  capil- 
lary force  draws  the  oil  up 
through  these  to  the  top  of 
the  wick,  where  it  is  burnt. 
When  one  end  of  a  cloth  is  put  into  water,  capillary  force 
draws  the  water  into  the.tubes  and  pores  of  the  cloth,  and 
the  whole  soon  becomes  wet.  In  the  same  way,  a  lump 
of  sugar,  or  other  porous  substance,  soon  becomes  wet 
throughout,  if  a  corner  of  it  is  put  into  water.  Blotting- 
paper  is  full  of  pores  into  which  the  capillary  force 
draws  the  ink.  The  use  of  a- towel  for  wiping  any  thing 
which  is  wet  depends  on  the  same  principle. 

34.  Strength  of  the  Capillary  Force. — When  a  piece 
of  cloth  is  wet,  it  is  quite  impossible  to  wring  or  squeeze 
it  dry.     This  shows  that  the  capillary  force  which  holds 
the  water   into   the   pores  of  the  cloth  is  very  strong. 
Many  other  facts  prove   the   strength  of  the   capillary 
force. 

35.  Capillary  Force  never  causes  a  Liquid  to  Jlow 
through  a   Tube.  —  If  we  take  a  glass  tube,  in  which 
the  capillary  force  will  raise  water  two  inches,  and  put 
the  tube  into  water  so  that  not  more  than  half  an  inch 
shall  be  above  the  surface,  the  water  will  not  overflow 
the  tube.     If,  however,  the  water  be  removed  as  soon 
as  it  comes  to  the  top,  more  will  rise  to  take  its  place. 

When  a  lamp  is  burning,  the  oil  is  passing  up  con- 
tinually through  the  wick,  because  it  is  burnt  as  soon 
as  it  reaches  the  top  ;  but  when  the  lamp  is  not  burning, 


ADHESION.  23 

the  oil  does  not  overflow  the  wick.  The  wick  of  an 
alcohol  lamp  must  be  covered  with  a  cap  when  not  in 
use,  or  the  alcohol  will  evaporate  as  fast  as  it  comes  to 
the  top  of  the  wick,  and  so  all  pass  out  of  the  lamp. 

36.  Adhesion  between  Solids  and  Gases.  —  If  a  piece 
of  boxwood  charcoal  be  put  into  a  jar  of  ammonia  gas 
over  mercury,  the  mercury  rapidly  rises  into  the  jar.* 
The  ammonia  gas,  then,  is  drawn  into  the  charcoal  by 
an  adhesive  force ;  showing  that  there  may  be  adhesion 
between  the  molecules  of  a  solid  and  those  of  a  gas. 
When  a  gas  is  taken  up  in  this  way  by  any  substance,  it 
is  said  to  be  absorbed. 

When  the  ammonia  gas  is  absorbed  by  the  charcoal, 
it  must  occupy  less  space  than  before.  When  a  gas  is 
absorbed  by  a  solid,  then,  the  repulsive  force  of  the  gas 
has  to  be  overcome.  We  know  that  cold  helps  to  over- 
come the  repulsive  force  (3).  Hence  we  should  expect 
that  a  solid  would  absorb  a  gas  when  cold  more  readily 
than  when  hot ;  and  this  is  found  to  be  true. 

Heat,  on  the  other  hand,  increases  the  repulsive  force. 
If  a  solid  which  has  absorbed  a  gas  be  heated,  the  repul- 
sive force  of  the  gas  is  increased,  so  that  it  finally  over- 
comes the  adhesion  of  the  solid  for  the  gas,  which  then 
leaves  the  solid.  The  charcoal  is  heated  before  it  is 
put  into  the  jar,  in  order  to  expel  the  air  from  its  pores. 

37.  Adhesion  between  Liquids  and  Liquids.  —  If  oil 
be  poured  upon  water,  the  oil,  which  is  the  lighter,  soon 
rises  to  the  top  and  remains  entirely  separate  from  the 
water.     If  alcohol,  which  is  also  lighter  than  water,  be 
poured  into  water,  the  two  will  thoroughly  mix.f     This 
proves  that  the  molecules  of  the  alcohol  adhere  to  those 
of  the  water,  and  that  this  adhesion  is  strong  enough  to 
overcome  the  cohesion  of  the  liquids. 

*  See,  in  Appendix,  Notes  on  Experiments. 

t  This  may  be  shown  better  by  using  colored  water. 


24 


ADHESION. 


Nearly  all  liquids  will  mix  when  poured  together, 
though  some  will  mix  much  more  readily  than  others. 

38.  Diffusion  of  Liquids.  —  Put  some  colored  alcohol 
into  a   tall  glass  jar,  and  then  with   a   funnel  carefully 
Fig.  12.  pour  in  some  water  (Figure  12).     The 

water  will  remain  for  a  time  at  the 
bottom  of  the  jar,  and  its  separation 
from  the  alcohol  will  be  sharply  de- 
fined ;  but  on  standing  a  few  days,  the 
two  liquids  will  become  thoroughly 
mixed.  This  mixing  of  liquids  on 
being  merely  brought  into  contact  is 
called  diffusion  of  liquids.  Different 
liquids  diffuse  into  each  other  at  very 
different  rates ;  while  some,  as  oil  and 
water,  will  not  diffuse  at  all. 

39.  Osmose  of  Liquids.  —  Fasten  a 
bladder  air-tight  to  the  end  of  a  long 
glass  tube,  fill  the  bladder  with  alcohol, 
and  put  it  into  a  vessel  of  water  (Fig- 
ure 13).  The  liquid  will  gradually  rise  in  the  tube; 
showing  that  the  water  has  passed  into  the  bladder. 
At  the  same  time  the  alcohol  passes  slowly  out  and  mixes 
with  the  water. 

The  mixing  of  liquids  when  separated  by  a  thin  mem- 
brane or  porous  substance  is  called  osmose  of  liquids. 

Liquids  do  not  mix  at  the  same  rate  when  separated 
by  a  thin  membrane  or  porous  substance  as  when  they 
mix  by  simple  diffusion.  The  rate  of  mixing  is  modified 
in  a  striking  manner  by  the  presence  of  the  membrane  or 
porous  substance. 

40.  Adhesion  between  Liquids  and  Gases.  —  Let  a 
small  glass  jar  inverted  over  mercury  be  filled  with 
ammonia  gas,  and  then  pour  some  water  over  the  sur- 
face of  the  mercury.  Raise  the  jar  carefully,  and,  as 


ADHESION. 


soon  as  its  mouth  comes  in  contact  with  the  water,  the 
latter  rises  and  completely  fills  the  jar.  The  ammonia 
is  absorbed  by  the  water,  Fig  I3> 

showing  that  the  mole- 
cules of  the  gas  adhere  to 
those  of  the  water. 

The  same  gas  is  not 
absorbed  with  equal  read- 
iness by  all  liquids.  Am- 
monia gas,  which  is  ab- 
sorbed so  greedily  by 
water,  is  not  absorbed  at 
all  by  mercury. 

41.  Cold  and  Pressure 
promote  Absorption.  — 
When  a  gas  is  absorbed 
by  a  liquid,  as  when  ab- 
sorbed by  a  solid  (36), 
the  molecules  are  brought 
nearer  together  and  the 
repulsive  force  overcome.  Both  cold  and  pressure,  as 
we  have  seen,  help  to  overcome  this  force ;  hence  they 
favor  the  absorption. 

The  effect  of  pressure  on  the  absorption  is  illustrated 
in  soda-water,  which  owes  its  agreeable  taste  mainly  to 
the  presence  of  carbonic  acid  gas  in  the  water.  Water 
and  carbonic  acid  are  brought  in  contact  in  the  fountain, 
and  subjected  to  very  great  pressure.  When  the  water  is 
drawn  from  the  fountain,  this  pressure  is  removed ;  and 
the  carbonic  acid  escapes  in  thousands  of  little  bubbles, 
causing  the  liquid  to  foam,  or  effervesce. 

Ordinary  aqua  ammonia  is  water  which  has  absorbed 
ammonia  gas.  If  it  be  heated,  the  gas  escapes.  The 
heat  increases  the  repulsive  force  of  the  gas,  and  thus 
overcomes  its  adhesive  force,  for  the  liquid.  THis  is  the 


26  ADHESION. 

ordinary  way  of  freeing  a  liquid  "from  a  gas  which  it 
has  absorbed. 

Common  spring-water  owes  much  of  its  pleasant  taste 
to  the  presence  of  carbonic  acid  and  other  gases  which  it 
Fig.  I4.  absorbs  from  the  air.    When 

this  water  is  boiled,  these 
gases  escape,  and  it  becomes 
very  insipid.  The  constant 
agitation  of  running  water 
helps  it  to  absorb  gases, 
_ since  it  is  thus  made  to  pre- 
sent more  surface  to  the  air. 
42.  Diffusion  of  Gases. — 
Two  bottles  are  connected 
by  a  long  glass  tube  (Fig- 
ure 14).  The  lower  bottle 
is  then  filled  with  carbonic 
acid  and  the  upper  with  hy- 
drogen gas,  which  is  very 
much  lighter  than  carbonic 
acid.  After  a  time,  the  hy- 
drogen will  be  found  to  have 
passed  down  and  mixed  with  the  heavier  carbonic  acid, 
and  the  carbonic  acid  to  have  mixed  with  the  hydrogen 
in  the  upper  bottle. 

We  may  prove  that  there  is  carbonic  acid  in  the  up- 
per bottle  by  pouring  lime-water  into  it  and  shaking  it. 
Carbonic  acid  makes  lime-water  milk-white,  while  hy- 
drogen has  no  effect  upon  it. 

The  mixing  of  gases  'when  brought  in  contact  is 
called  diffusion  of  gases. 

Different  gases  diffuse  into  each  other  at  very  different 
rates.  As  a  general  thing,  the  more  the  gases  differ  in 
weight,  the  more  rapidly  they  diffuse  into  each  other. 

43.  Osmose  of  Gases.  —  A  long  glass  tube  is  fastened 
air-tight,  by  means  of  a  cork  and  sealing-wax,  into  the 


ADHESION. 


open  end  of  an  unglazed  porcelain  cup.  The  cup  is  then 
held  so  that  the  end  of  the  tube  dips  beneath  the  sur- 
face of  water,  and  a  large  F'is-  is- 
bell-jar  of  hydrogen  is  held 
over  the  cup.  There  is  an 
instant  rush  of  bubbles  from 
the  end  of  the  tube  up  through 
the  water,  showing  that  the 
hydrogen  has  passed  through 
the  pores  of  the  cup  and  mixed 
with  the  air  inside.  Remove 
now  the  jar  of  hydrogen,  and 
the  water  at  once  rises  in  the 
tube ;  showing  that  the  hy- 
drogen inside  the  cup  has 
passed  out  through  the  pores 
to  mix  with  the  air  outside. 

The  mixing  of  gases  when 
separated  by  a  porous  sub- 
stance or  thin  membrane  is 
called  osmose  of  gases. 


SUMMARY. 

Adhesion  is  the  force  which  holds  together  molecules 
of  different  kinds. 

It  acts  between  molecules  of  solids  and  solids,  solids 
and  liquids,  solids  and  gases ;  also  between  liquids  and 
liquids,  and  liquids  and  gases.  It  is  doubtful  whether 
it  acts  between  the  molecules  of  different  gases. 

The  adhesion  between  two  solids  is  sometimes  stronger 
than  the  cohesion  of  the  solids  themselves.  (25.) 

There  are  three  cases  of  adhesion  between  solids  and 
liquids :  — 


28  ADHESION. 

ist,  When  the  adhesion  is  not  strong  enough  to  over- 
come the  cohesion  of  the  liquid,  and  the  liquid  cannot 
ivet  the  solid. 

2cl,  When  it  is  strong  enough  to  overcome  the  cohe- 
sion of  the  liquid,  and  the  liquid  can  wet  the  solid. 

3d,  When  it  is  strong  enough  to  overcome  the  cohe- 
sion of  the  solid,  and  the  liquid  can  dissolve  the 
solid.  (26-30.) 

Heat  generally  promotes  solution,  since  it  helps  to 
overcome  the  cohesion  of  the  solid.  (31.) 

Capillary  force  acts  upon  liquids  in  tubes.  Liquids 
which  can  wet  a  tube  are  drawn  into  it,  while  liquids 
which  cannot  wet  it  are  driven  out  of  it.  The  finer  the 
tube,  the  more  marked  is  the  capillarity.  (32.) 

The  capillary  force  is  very  strong ;  but  acting  al6ne  it 
never  makes  a  liquid  flow  through  a  tube.  (33,  35.) 

When  a  gas  is  absorbed  by  a  solid  or  by  a  liquid,  the 
adhesive  force  between  the  molecules  of  the  solid  or 
liquid  and  those  of  the  gas  must  be  strong  enough  to 
overcome  the  repulsive  force  of  the  gas.  (36,  40.) 

Heat  hinders  absorption,  since  it  increases  the  repul- 
sive force  between  the  molecules  of  the  gas.  Hence 
gases  absorbed  by  solids  or  liquids  can  be  separated 
from  them  by  means  of  heat.  (36,  41.) 

The  same  gas  is  absorbed  by  some  solids  or  liquids 
more  readily  than  by  others.  (40.) 

The  adhesive  force  between  the  molecules  of  different 
liquids  causes  the  liquids  to  mix.  The  mixing  of  liquids 
on  mere  contact  with  each  other  is  called  diffusion  of 
liquids.  (38.) 

Liquids  also  mix  when  separated  by  a  thin  membrane 
or  porous  substance.  This  mixing  is  called  osmose  of 
liquids.  (39.) 

Gases,  like  liquids,  mix  by  diffusion  and  by  osmose. 

(42>  43-) 


MECHANICS. 

PRESSURE. 
WEIGHT. 

44.  Matter  is  acted  upon  by  Gravity.  —  Thus  far 
we  have  been  dealing  with  forces  which  act  between  the 
molecules  of  matter.  We  are  now  to  study  a  force  which 
acts  between  masses  of  matter.  When  a  stone  falls  to 
the  earth,  xit  is  because  this  force  acts  between  the  stone 
and  the  earth  to  draw  them  together.  We  know  that  the 
moon  is  continually  moving  round  the  earth.  Were  it  not 
for  this  force  acting  between  these  two  great  masses,  the 
moon  would  flyt>fFin  a  straight  line,  and  we  should  never 
see  her  again.  As  it  is,  while  moving  onward,  she  is  all 
the  while  falling  towards  the  earth,  and  her  path  is  thus 
bent  from  a  straight  line  into  a  curve.  It  is  found  that  the 
strength  of  this  force  diminishes  as  the  square  of  the 
distance  increases;  that  is,  at  double  the  distance 'its 
strength  is  one-fourth,  at  thrice  the  distance  one-ninth, 
and  so  on.  It  can  be  proved  that  the  moon  obeys  this 
law  and  falls  towards  the  earth  just  as  fast  as  a  stone 
•would  fall  if  it  were  as  far  off. 

It  is  the  same  force  which  keeps  the  earth  in  its  path 
about  the  sun,  and  which  guides  all  the  stars  of  heaven 
in  their  appointed  courses.  It  acts  upon  every  mass  of 
matter  in  the  universe,  from  the  dust  that  floats  in  the 
air,  to  moons  and  planets  and  suns,  compared  with 


MECHANICS. 


whose  vast  bulk  this  earth  of  ours  is  a  floating  particle 

of  dust.     This  force  is  called  gravity.* 

!    45.    Weight.  —  When  a  stone  is  held  in  the  hand,  it 

is  felt  to  press  downward.     This   is  because  gravity  is 

drawing  it  toward*  the  earth.     The  downward  pressure 

which  gravity   causes   a   body   to   exert   is    called    its 

weight. 

When  different  bodies,  as  iron  and  wood,  are  taken  in 
the  hand,  it  is  easy  to  feel  that  some  are  heavier  than 
others ;  but  it  is  not  so  easy  to  tell  exactly  how  much 
they  differ  in  weight. 

46.   The  Spring  Balance.  —  But  the  weight  of  a  body 
may  be  made   to   bend   a    spring,   and,  when   different 
bodies  are  made  to  bend  the  same  spring,  we  can  readily 
tell  how  much  heavier  one  is  than  another  by  seeing  how 
much  more  it  bends  the  spring.     If  it  bends  the  spring 
twice  as  much,  it  is  twice  as  heavy ;  and  if  three  times  as 
Fig.  16.         much,  it  is  thrice  as  heavy.     An  instrument 
^          ^     for  finding  the  weight  of  a  body  in   this 
way  is  called  a  spring  balance;  and  one 
form  of  it  is  shown  in  Figure  16.     It  con- 
sists of  a  steel  spring  wound  into  a  coil. 
One  end  of  this  coil  is  fastened  to  a  ring, 
and  the  other  to  a  hook.     The  body  to  be 
•weighed  is  fastened  to  the  hook,  and  the 
whole  raised  by  the  ring.     The  weight  of 
the    body   straightens    or    draws    out    the 
spring.     A  pointer  moving  over  a  plate  in 
front,  which  is   divided   into   equal   parts, 
shows  how  much  the  spring  has  been  drawn  out.     A 
body  which  will  straighten  the  spring  a  certain  amount 

*  We  have  already  referred  (see  foot-note  on  page  4)  to  the 
analogy  between  the  molecules  in  a  mass  and  the  great  masses 
in  solar  and  stellar  systems.  The  analogy  can  be  extended  to 
the  forces  acting  among  the  units  which  make  up  the  two. 


MECHANICS. 


is  said  to  weigh  a  pound ;  one  which  will  straighten  it 
half  as  much,  half  a  pound ;  one  fourth  as  much,  a 
quarter  of  a  pound ;  twice  as  much,  two  pounds ;  and 
so  on. 

47.  The  Balance. — An- 
other   instrument   for  rind- 
ing the  weight   of  a   body 
is  called  a  balance,  and  is 
shown   in    Figure    17.      It 
consists  of  a  bar  turning  on 
a  pivot  in  the  centre,  and 
having    pans    hung     from 

each  end  for  holding  the  body  to  be  weighed  and  the 
'weights,  which  are  pieces  of  iron  or  brass  whose  weight 
is  known.  The  body  to  be  weighed  is  placed  in  one 
pan,  and  weights  are  put  into  the  other  until  they  balance 
it.  The  body  weighs  as  much  as  the  weights  used. 

48.  The    Steelyard.  —  Yet 
another  weighing  instrument  is 
the  steelyard,  shown  in  Figure 
1 8.     It  differs  from  the  balance 
in   having   one    long    and    one 
short  arm.     The  weight  P  can 
be  moved  upon  the  long  arm, 
and   shows   the  weight   of  the 
body  by  its  distance  from  C. 


Fig.  18. 


THE   CENTRE  OF  GRAVITY. 

49.  The  Centre  of  Gravity.  —  The  centre  of  gravity 
of  a  body  is  a  point  such  that  the  force  of  gravity  act- 
ing upon  the  part  of  the  body  on  one  side  of  this  point 
always  balances  the  force  of  gravity  acting  upon  the 
part  on  the  opposite  side,  no  matter  how  the  body  may 
be  placed. 


32  MECHANICS. 

50.  The  Centre  of  Gravity  is  not  always  in  the  Body 
itself.  —  If  a  straight  strip  of  metal  or  wood  be  fastened 
to  the  sides  of  a  ring  so  as  to  pass  through  its  centre,  it 
will  be  found  that  the  ring  will  rest  in  any  position  when 
the  centre  is  supported ;  and  that  it  will  not  thus  remain 
at  rest  on  any  other  point.     The  centre  of  gravity,  then, 
of  a.  ring  which  is  exactly  alike  throughout  its  whole 
extent  is  at  the  centre  of  the  ring.     If  one  part  of  the 
ring  is  heavier  than  the  other,  the  centre  of  gravity  will 
be. found  to  be  between  the  centre  and  the  heavier  part. 

When  two  balls  of  the  same  weight 
are  connected  by  a  straight  rod  (Fig- 
ure 19),  the  centre  of  gravity  will  be 
found  to  be  at  the  centre  of  the  rod. 
If  one  ball  be  twice  as  heavy  as  the 
other,  the  centre  of  gravity  will  be  in 
the  rod  at  a  point  twice  as  near  the 
heavier  ball  as  the  lighter  ball.  If  the  heavier  ball  be 
three  times  the  weight  of  the  lighter  ball,  the  centre  of 
gravity  will  be  thrice  as  near  this  ball  as  the  other. 

If  the  balls  are  connected  by  a  curved  rod,  the  centre 
of  gravity  will  no  longer  be  in  the  rod,  but  in  a  straight 
line  which  joins  the  balls.  Its  distance  from  the  balls 
will  be  as  explained  above. 

51.  Equilibrium.  —  When  a  body  is  at  rest,  it  is  said 
to  be  in  equilibrium.    When  it  is  at  rest  in  such  a  posi- 
tion that  on  being  slightly  disturbed  it  again   returns 
to  this  position,  it  is  said  to  be  in  stable  equilibrium. 
When   it  is   at   rest   in  such  a  position  that  on  being 
slightly   disturbed  it  seeks  a  new  position  of  rest,  it 
is  said  to  be   in   unstable  equilibrium.     Wlien  a  body 
remains  at  rest  equally  well  in  any  position,  it  is  said 
to  be  in   indifferent  equilibrium.  >r- 

52.  The  Centre  of  Gravity  always  seeks  the  Lowest 
Point.  —  In  every  case,  it  will  be  found  that  the  centre  of 

"\ 


MECHANICS. 


33 


gravity  of  a  body  seeks  the  lowest  position  that  it  can 
take.  Hence,  when  a  body  is  so  situated  that  its  centre 
of  gravity  is  raised  by  tipping  it  in  any  direction,  it  is  in 
stable  equilibrium  ;  when  any  disturbance  of  the  body 
tends  to  lower  its  centre  of  gravity,  it  is  in  unstable 
equilibrium ;  when,  on  being  disturbed,  its  centre  of 
gravity  neither  rises  nor  falls,  it  is  in  indifferent 
equilibrium.  ^L.  4 

In  Figure  20,  g  e  shows  the  path  which  the  centre  of 
gravity  g  must  take  when  the  body  is  tipped.  Until  g 
reaches  the  point  e,  the  body  tends  to  go  back,  because 
in  so  doing  the  centre  of  gravity  would  fall ;  but  as  soon 
as  g  passes  e,  the  body  tends  to  go  over,  because  in  so 
doing  the  centre  of  gravity  would  fall,  h  e  shows  how 
mucn  the  centre  of  gravity  must  be  raised  to  overturn  the 
body ;  and  this  distance  is  seen  to  be  greater  when  the 

Fig.  20. 


V 

^.       c 

&*+ 

n~" 

I           A 

z 

X 

*-.       ^s 

s 

tody  is  resting  on  the  side  a  b  than  when  it  is  resting  on 
the  side  b  c.  It  will  be  found  that  much  more  force  will 
be  required  to  overturn  it  in  the  latter  case  than  in  the 
former.  Hence,  the  more  the  centre  of  gravity  of  a 
body  has  to  be  raised  in  order  to  overturn  it,  the  more 
stable  its  equilibrium. 

It  will  also  be  seen  from  Figure  20  that  the  broader 
the  base  of  a  body  compared  with  its  height,  the  more 
stable  its  equilibrium. 

3 


34 


MECHANICS. 


If,  however,  the  body  is  not  upright,  it  may  be  in  un- 
stable equilibrium  even  when  the  base  is  broad.  In 
Figure  2i,g  e  is  the  path  which  the  centre  of  gravity^ 

Fig.  21. 

(lr~ ,  O.  O, -,  I 


must  take  when  the  body  a  b  c  d  is  overturned ;  and  it 
Fig.  22.          will  be  seen  that,  as  soon  as  g  is  moved 
at  all  in  the  direction  g  e,  it  begins  to  fall, 
and  the  body  will  go  over.     In  the  body 
/  m  n  a,  the  centre  of  gravity  g  is  not  sup- 
ported, and  the  body  will  fall  over  of  itself. 
It  is  evident,  then,  that  a  body  may  lean 
and  yet   be  in  equilibrium,   provided  the 
centre  of  gravity  is    directly   over   any 

point  of  the  base.    If  this  point  be  well  Fjg.  23. 

within  the  base,  the  equilibrium  m 

be  very  stable,  as   in   the  case    of  the 

famous  leaning  tower  at  Pisa. 

On  the  other  hand,  a  body  may  be 

in   stable   equilibrium   even    when  the 

base  is  very  narrow.    Thus  a  cork  may 

rest  upon  the  point  of  a  needle,  and  yet 

be  in  stable  equilibrium.     This  may  be 

proved  by  sticking  two  forks   into  the 

cork,    as    shown    in  Figure    22.       The 

forks  bring  the  centre  of  gravity  below 

the  point  of  support,  so  that  the  cork 

cannot  be  tipped   without   raising   the 


•  MECHANICS. 


35 


centre  of  gravity.  In  the  same  way,  the  image  in  Fig- 
ure 23  is  balanced  on  its  toe  by  means  of  the  two 
heavy  balls  beneath.  So,  too,  in  F- 

the  u  prancing  horse  "  (Figure  24) 
the  centre  of  gravity  is  brought 
below  the  point  of  support  by  the 
leaden  ball  at  the  end  of  the 
curved  rod. 

53.  How  to  find  the  Centre  of 
Gravity  of  a  Solid.  —  When  a 
stone,  as  in  Figure  25,  is  hung  by 
the  cord  A,  the  centre  of  gravity 
must  be  directly  under  the  point 
of  support ;  that  is,  somewhere  in  the  line  A  B.  If  the 
same  stone  be  hung  by  the  cord  C,  its  centre  of  gravity 
must  still  be  below  the  point  of  support,  somewhere  in 
the  line  C  J).  Since  the  centre  of  gravity  is  in  both  the 


Fig.  25. 


lines  A  B  and  C  D,  it 
must  be  at  the  point  G, 
where  they  cross. 

To  find  the  centre  of 
gravity  of  a  solid,  then, 
suspend  it  from  any. 
point  of  its  surface  by 
means  of  a  cord,  and 
notice  the  direction 
which  the  cord  takes. 
Then  suspend  it  from 
^another  point,  and  again  notice  the  direction  of  the 
cord.  .  The  point  within  the  body  where  lines  drawn 
in  these  directions  would  cross  each  other  will  be  the 
centre  of  gravity. 

Of  course,  if  the  solid  be  of  regular  shape  and  of 
uniform  density,  the  centre  of  gravity  is  at  the  centre 
of  magnitude. 


36  MECHANICS. 


SUMMARY. 

Matter  is  acted  upon  by  gravity,  which  gives  it 
-weight.  (44,  45.) 

*  The  weight  of  bodies  may  be  found  by   the  spring 
balance  (46),  the  balance  (47),  or  the  steelyard  (48). 

A  point  can  always  be  found  such  that  the  force  of 
gravity  acting  upon  the  part  of  a  body  to  the  right  of  it  is 
always  balanced  by  the  force  of  gravity  acting  upon  the 
part  to  the  left  of  it,  no  matter  in  what  position  the  body 
may  be  placed.  This  point  is  called  the  centre  of  grav- 
ity, and  sometimes  lies  within  a  body  and  sometimes 
without  it.  (49,  50.) 

A  body  at  rest  is  in  equilibrium.  Its  equilibrium 
.may  be  stable,  unstable,  or  indifferent.  (51.) 

The  centre  of  gravity  always  seeks  the  lowest  position 
which  it  can  take.  The  stability  of  equilibrium  depends 
upon  the  position  of  the  centre  of  gravity,  and  upon  how 
much  it  must  be  raised  to  overturn  the  body.  (52.) 

The  centre  of  gravity  of  a  solid  may  be  found  by  sus- 
pending the  body  from  two  points  of  its  surface.  (53.) 


PRESSURE  OF  LIQUIDS. 

54.  How  to  find  the  Weight  of  a  Liquid.  —  If  we 
weigh  a  cup  and  then  fill  it  with  water  and  weigh  it 
again,  we  shall  find  that  it  weighs  more  in  the  second 
case.  Liquids,  as  well  as  solids,  are  acted  upon  by  grav- 
ity, which  causes  them  to  exert  a  downward  pressure. 
The  weight  of  the  water  in  the  cup  is  the  weight  of  the 
cup  when  full  of  water  less  the  weight  of  the  empty  cup. 
If  the  cup  be  filled  with  quicksilver,  it  will  be  found  to 
weigh  much  more  than  when  filled  with  water ;  show- 
ing that  some  liquids  are  heavier  than  others. 


MECHANICS. 


37 


55.  Liquids 'when  acted  upon  by  Gravity-press,  not 
only  downward,  but  also  upward  and  sideways.  —  Fix 
a  long  tube  into  the  top  of  a  wooden  cask,  and  put  a 
stop-cock  into  the  top,  and  another  into  the  side  of  the 
cask.     Fill  the  cask  and  the  tube  with  water,  and  open 
the  stop-cocks,  and  the  water  will  be  driven  out  of  both. 
This  shows  that  the  water  in  the  cask  presses  upward 
and  sideways  as  well  as  downward. 

The  pressure  which  liquids  exert  sideways  is  called 
lateral  pressure. 

56.  The    Upward,   Downward,   and  Lateral  Pres- 
sures are  equal  for  the  same  Depth  of  Liquid.  —  In 
Figure  26,  we  have   a  glass  vessel, 

into  the  top  of  which  are  inserted 
three  glass  tubes  of  exactly  the  same 
size,  with  their  mouths  at  the  same 
distance  from  the  bottom.  One  of 
these  tubes  opens  downward,  one 
upward,  and  one  sideways.  If  we 
fill  the  vessel  with  water,  through 
the  funnel,  the  liquid  rises  to  the 
same  height  in  all  three  tubes.  Now  it  is  the  upward 
pressure  which  causes  it  to  rise  in  the  tube  opening 
downward,  the  lateral  pressure  which  causes  it  to  rise 
in  the  tube  opening  sideways,  and  the  downward  pres- 
sure which  causes  it  to  rise  in  the  tube  opening  upward  ; 
and  since  the  tubes  are  all  of  the  same  size,  and  the  water 
rises  to  the  same  height  in  each,  these  pressures  are  all 
evidently  equal. 

57.  The   Upward,   Downward,   and  Lateral  Pres- 
sures of  a  Liquid  increase  with  the  Depth,  but  are  not 
altered  by  the  Size  or  Form  of  the  Vessel  which  holds 
the  Liquid.  —  The  more  water  we  pour  into  the  vessel, 
in  Figure  26,  the  higher  the  water  rises  in  the  tubes. 
The  upward,  downward,  and  lateral  pressures  increase 


38  MECHANICS. 

with  the  depth  of  the  liquid,  since  the  lower  layers  of 
molecules  are  themselves  acted  upon  by  gravity,  and  have 
also  to  sustain  the  pressure  of  the  water  above  them. 

That  the  pressure  is  independent  of  the  size  and  shape 
of  the  vessel  is  seen  when  vessels  of  different  sizes*  and 
shapes  are  connected  (Figure  27),  and  a  liquid  is  poured 
into  one  of  them.  It  rises  to  the  same  height  in  all. 

Fig.  27. 


58.  When  a  closed   Vessel  is  filled  'with   a  Liquul, 
and  any  additional  Pressure  is  brought  to  bear  on  any 
Particle  of  this  Liquid,  every  Particle  is  made  to  exert 
the  same  additional  Pressure,  upward,  downward,  and 
sideways.  —  Suppose  the  four  tubes  in  Figure  26  are  all 
of  Exactly  the   same  size,  and  that  the  vessel  is  full  of 
water.     Pour  water  into  the  left-hand  tube  until  it  rises 
to  the  line  c  d.     The  water  rises  in  all  the  tubes  to  the 
same  height.    The  water  poured  into  the  first  tube  brings 
an  additional  pressure  to  bear  upon  the  particles  of  water 
at  its  mouth,  and  it  is  the  additional  pressure  which  the 
particles  at  the  end  of  the  other  tubes  are  made  to  exert 
that  causes  the  water  to  rise  in  them.     Since  they  are  all 
of  the  same  size,  there  must  be  the  same  number  of  parti- 
cles at  the  end  of  each ;   therefore,  the  particles  at  the 
end  of  the  three  tubes  are  made  to  exert  the  same  ad- 
ditional pressure  upward,  downward,  and  sideways,  as 
that  brought  to  bear  upon  the  particles  at  the   end  of 
the  left-hand  tube. 

59.  The  Hydrostatic  Press.  —  It  follows,  from  what 
has  just  been  shown,  that  by  means  of  a  liquid  a  small 
pressure  upon  a  small  surface  may  be  made  to  exert  a 


MECHANICS. 


39 


great  pressure  upon  a  large  surface.     In  Figure  28,  we 
have  two  cylinders,  with  a  plunger,  or  piston,  in  each. 

Fig.  28. 


Suppose  that  the  surface  of  the  larger  piston,  P,  is  thirty 
times   that  of  the  smaller,  p;   if  the  latter  is  pressed 

Fig.  29- 


downward  by  a  weight  of  one  pound,  an  upward  pres- 


4o 


MECHANICS. 


sure  of  one  pound  will  be  brought  to  bear  upon  each 
portion  of  the  surface  of  P  equal  to  that  of  p.  The 
whole  upward  pressure  on  P  will  then  be  thirty  times 
the  downward  pressure  on  p.  If  the  surface  of  P  had 
been  sixty  times  that  of  p,  one  pound  on  the  latter  would 
have  balanced  sixty  on  the  former  ;  and  so  on. 

Advantage  is  taken  of  this  fact  in  the  construction  of 
the  hydrostatic  press,  shown  in  Figures  29  and  30.  The 
two  cylinders  A  and  JB  are  connected  by  the  pipe  d. 

Fig.  30. 


The  piston  #,  in  the  small  cylinder  A,  is  worked  by  the 
handle  0,  and  forces  water  into  the  large  cylinder  j??, 
where  it  presses  up  the  piston  C.  If  the  end  of  the 
piston  C  is  1,000  times  as  large  as  that  of  the  piston  a, 
a  pressure  of  2  pounds  on  a  would  exert  a  pressure  of 
2,000  pounds,  or  one  ton,  upon  C.  If  a  man  in  working 
the  handle  O  forces  down  the  piston  a  with  a  pressure 
of  50  pounds,  he  would  bring  to  bear  upon  C  a  pressure 
of  25  tons. 

This  press  is  used  for  pressing  cotton,  hay,  cloth,  etc., 
into  bales,  for  extracting  oil  from  seeds,  testing  cannon, 
boilers,  etc.,  and  for  raising  ships  out  of  the  water. 


MECHANICS. 

60.  Springs  and  Artesian  Wells. — -^All  natural  col- 
lections of  water  illustrate  the  tendency  of  a  liquid  to 
find  its  level.  Thus,  the  Great  Lakes  of  North  America 
may  be  regarded  as  a  number  of  vessels  connected  to- 
gether, and  hence  the  waters  tend  to  maintain  the  same 
level  in  all.  The  same  is  true  of  the  source  of  a  river 
and  the  sea,  the  bed  of  the  river  connecting  the  two  like 
a  pipe. 

Springs  illustrate  the  same  fact.  The  earth  is  com- 
posed of  layers,  or  strata,  of  two  kinds ;  those  through 
which  water  can  pass,  as  sand  and  gravel,  and  those 
through  which  it  cannot  pass,  as  clay.  The  rain  which 
falls  on  high  ground  sinks  through  the  soil  until  it  reaches 
a  layer  of  this  latter  kind,  and  along  this  it  runs  until  it 
finds  some  opening  through  which  it  flows  as  a  spring. 

It  is  the  same  with  Artesian  wells.  These  wells  de- 
rive their  name  from  the  Province  of  Artois  in  France, 
the  first  part  of  Europe  where  they  became  common.  It 
would  seem,  however,  that  wells  of  the  same  kind  were 
made  in  China  and  Egypt  many  centuries  earlier. 

In  Figure  3 1 ,  suppose  A  B  and  C  D  to  be  two  strata  of 
clay,  and  K  K  to  be  a  stratum  of  sand  or  gravel  between 


Fig. 


them.  The  rain  falling  on  the  hills  on  either  side  will 
filter  down  through  this  sand  or  gravel,  and  collect  in 
the  hollow  between  the  two^ strata  of  clay,  which  prevent 


42  MECHANICS. 

its  escape.  If  now  a  hole  be  bored  down  to  K  K,  the 
water,  striving  to  regain  its  level,  will  rise  to  the  surface 
at  H,  or  spout  out  to  a  considerable  height  above  it. 

An  Artesian  well  in  Paris  has  a  depth  of  548  metres, 
or  about  i  ,800  feet,  and  the  water  flows  out  at  the  rate 
of  656  gallons  a  minute,  or  nearly  a  million  gallons  a 
day.  One  in  this  country,  at  St.  Louis,  is  2,199  feet 
deep,  and  affords  75  gallons  a  minute. 

61.  A  Body  is  buoyed  up  ivhen  placed  in  a  Liquid. 
—  If  a  stone  be  weighed  under  water,  it  will  seem  to  be 
lighter  than  when  weighed  in  the  air. 

We  have  already  seen  that,  at  the  same  depth  in  a 
liquid  the  upward  and  downward  pressures  are  equal, 
but  that  these  pressures  increase  with  the  depth.  The 
bottom  of  the  stone  in  the  above  experiment  being  deeper 
in  the  water  than  the  top,  the  upward  pressure  of  the 
Fig.  32.  water  against  the  bottom  of  the  stone 

is  greater  than  the  downward  pressure 
upon  the  top  of  the  stone.  The  stone 
is  accordingly  lifted  up  a  little  when 
plunged  under  water,  and,  being  thus 
buoyed  up,  seems  to  be  lighter  than 
in  the  air. 

62.  A  Body  is  buoyed  up  in  Water 
by  a  force  just  equal  to  the  Weight 
of  the  Water  'which  it  displaces.  —  In 
Figure  32,  A  is  a  cup  into  which  the 
cylinder  B  exactly  fits.  This  cup,  then, 
will  hold  just  as  much  water  as  B 
displaces  when  under  water.  Hang 
this  cup  and  cylinder  to  the  hydrostatic 
balance,  and  balance  it  with  weights. 
Immerse  the  cylinder  B  in  a  vessel  of 
water,  and  we  find  that  it  is  more  than 
balanced  by  the  weights.  Now  care- 


MECHANICS.  43 

*• 

fully  fill  the  cup  A  with  water  from  the  vessel,  and 
the  cup  and  cylinder  are  seen  to  be  again  just  bal- 
anced by  the  weights.  This  shows  that  a  body  when 
immersed  in  water  is  buoyed  up  by  a  force  just  equal 
to  the  weight  of  the  water  which  it  displaces. 

Of  course,  if  a  solid  weighs  exactly  as  much  as  the 
water  it  displaces  when  fully  immersed,  it  will  neither 
rise  nor  sink  in  the  water.  If  it  weighs  more  than  the 
water  it  displaces,  it  will  sink ;  if  less,  it  will  rise. 
When  a  body  floats  upon  the  water,  it  displaces  exactly 
its  own  weight  of  water.  It  is  well  known  that  a  lump 
of  iron  will  sink,  but  the  same  lump  of  iron  may  be 
hammered  out  into  a  vessel  which  will  displace  its  own 
weight  .of  water  without  being  wholly  immersed. 

In  this  way,  ships  may  be  made  of  iron  which  will 
float  upon  water  as  well  as  ships  made  of  wood. 

SPECIFIC  GRAVITY. 

63.  Substances  vary  in  Density.  —  The  same  bulks 
of  different  solids  and  liquids  are  found  to  be  very  differ- 
ent in  weight.     A  substance  which  weighs  more,  bulk 
for  bulk,  than  another  substance  is   said   to   be   more 

dense,  or  to  have  a  greater  density.  It  is  often  desirable 
to  know  the  relative  weights  of  the  same  bulks  of  bodies 
which  vary  in  density.  In  such  cases,  it  is  convenient 
to  compare  the  weight  of  each  substance  with  the  weight 
of  some  given  substance ;  and  water  is  taken  for  this 
purpose.  The  weight  of  a  given  substance  compared 
with  the  weight  of  the  same  bulk  of  water  is  called  its 
specific  gravity. 

64.  Specific  Gravity  of  Solids.  —  To  find  the  specific 
gravity  of  a  solid  or  liquid,  we  must  know  the  weight  of 
the  substance  and  that  of  the  same  bulk  of  water. 

The  weight  of  a  bulk  of  water  equal  to  that  of  the 


44 


MECHANICS. 


solid  can  be  found  by  weighing  the  solid  in  water,  and 
subtracting  its  weight  in  water  from  its  weight  in  air. 
The  difference  of  these  weights  is  (62)  just  equal  to  the 
weight  of  the  water  it  displaces ;  and  this  is,  of  course, 
a  bulk  of  water  just  equal  to  its  own  bulk. 

Hence  we  have  the  rule :  divide  the  weight  of  the 
body  by  its  loss  of  weight  in  water,  and  the  quotient 
will  be  its  specific  gravity. 

If  the  body  will  not  sink  in  water,  fasten  it  to  a  piece 
of  iron  or  other  substance  which  will  make  it  sink,  and 
find  the  weight  lost  by  the  two  in  water.  The  difference 
between  what  Jboth  lose  and  what  the  iron  weighed  alone 
loses  will,  of  course,  be  what  the  lighter  body  loses. 

65.  Specific  Gravity  of  Liquids.  —  The  specific  grav- 
ity of  liquids  is  most  conveniently  found  by  means  of  an 
instrument,  shown  in  Figure  33,  called  a  hydrometer. 

Fig-  33- 


It  consists  of  a  hollow  glass  cylinder,  with  a  stem  and 
scale-pan  above,  and  a  small  bulb  filled  with  mercury 
below,  by  which  it  is  made  to  float  upright  in  a  liquid. 
The  instrument  is  placed  in  water,  and  weights  are 
added  until  it  sinks  to  a  point  marked  upon  the  stem. 


MECHANICS.  45 

The  weight  of  the  hydrometer,  together  with  the  weights 
in  the  pan,  is  equal  to  the  weight  of  the  water  displaced 
(62).  If  now  the  instrument  be  placed  in  a  liquid  of  dif- 
ferent density,  as  alcohol,  and  made  to  sink  by  weights 
to  the  mark  on  the  stem,  the  weight  of  an  equal  bulk  of 
that  liquid  can  be  found.  Of  course,  the  specific  grav- 
ity of  the  liquid  will  be  the  weight  of  the  liquid 
divided  by  the  weight  of  the  water. 

A  more  common  form  of  hydrometer  is  shown  in  Fig- 
ure 34.  It  consists  of  a  glass  tube  and  bulb  loaded  with 
mercury  at  the  bottom.  This,  when  put  into  a  liquid  in 
which  it  will  float,  always  displaces  just  its  own  weight 
(62).  It  is  first  put  into  pure  water,  and  the  point  to 
which  it  sinks  is  marked  upon  the  stem.  If  it  be  now 
put  into  a  liquid  of  less  density,  it  will  sink  deeper ;  if 
into  one  of  greater  density,  it  will  not  sink  so  deep.  By 
means  of  the  scale  on  the  stem,  the  specific  gravity  of  the 
liquid  into  which  it  is  put  is  indicated. 

Another  way  to  find  the  specific  gravity  of  a  liquid  is 
the  following :  Fill  a  small  bottle  with  water,  and  then 
with  the  liquid,  and  find  the  weight  of  each ;  then  di- 
vide the  weight  of  the  liquid  by  the  weight  of  the  water, 
and  the  quotient  will  be  the  specific  gravity  required. 

A  specific  gravity  bottle  is  a  bottle  which  is  made  to 
hold  a  definite  weight  of  water,  as  1,000  grains.  If  it 
holds  790  grains  of  alcohol,  the  specific  gravity  of  the 
alcohol  is  .79 ;  if  it  holds  1,860  grains  of  sulphuric  acidr 
the  specific  gravity  of  the  acid  is  i  .86 ;  and  so  on. 

Again,  since  the  weight  which  a  body  loses  when  im- 
mersed in  a  liquid  is  equal  to  the  weight  of  its  own  bulk 
of  that  liquid  (23),  we  can  find  the  specific  gravity  of 
a  liquid  by  dividing  the  weight  which  a  body  loses 
in  that  liquid  by  the  weight  which  it  loses  in  water. 
Thus,  if  a  piece  of  copper  loses  200  grains  when  weighed 
in  water,,  and  158  grains  when  weighed  in  alcohol,  the 


46  MECHANICS. 

specific  gravity  of  the  alcohol  is  equal  to  158  divided  by 
200,  or  .79. 


SUMMARY. 

Liquids  have  weight,  and  press  upward,  downward, 
and  sideways.  (54,  55.) 

The  upward,  downward,  and  lateral  pressures  are 
always  equal  for  the  same  depth  of  the  liquid.  (56.) 

These  pressures  increase  with  the  depth  of  the  liquid, 
but  are  not  altered  by  the  size  or  shape  of  the  vessel. 

(57-) 

When  any  pressure  is  brought  to  bear  upon  one  parti- 
cle of  a  liquid,  every  particle  is  made  to  press  with  the 
same  force  upward,  downward,  and  sideways.  (58.) 

On  this  account,  when  a  small  force  acts  upon  a  few 
particles  of  a  liquid,  an  enormous  force  may  be  brought 
to  bear  on  a  large  surface  in  contact  with  the  same  liq- 
uid. This  is  illustrated  by  the  hydrostatic  press.  (59.) 

Springs  and  Artesian  wells  illustrate  the  tendency  of 
water  to  seek  a  level  in  connected  vessels.  (60.) 

A  body  is  buoyed  up  in  water  by  a  force  equal  to  the 
weight  of  the  water  which  it  displaces.  (61,  62.) 

The  specific  gravity  of  a  solid  or  liquid  is  the  weight 
of  the  solid  or  liquid  compared  with  the  weight  of  the 
same  bulk  of  water.  (63.) 

To  find  the  specific  gravity  of  a  solid,  divide  its  weight 
by  its  loss  of  weight  in  water.  (64.) 

The  hydrometer  is  an  instrument  for  finding  the  spe- 
cific gravity  of  liquids.  (65.) 


e  Problems  on  the  pressure  and  weight  of  liquids, 
given  in  the  Appendix,  are  intended  to  be  used  at  this  point. 


MECHANICS. 


47 


THE  PRESSURE  OF  GASES. 

66.  Gases  have    Weight.  —  Weigh  very  carefully  a 
thin  copper  globe  when  filled  with  air;  then  weigh  it 
again  after  exhausting  the  air  from  it,  and  it  will  be 
found  to  weigh  less  than  before.     This  shows  that  air 
has  weight,  and  the  same  is  true  of  all  other  gases. 

67.  Gases,  like  Liquids,  press  upward,  downward, 
and  sideways.  —  Figure  35  represents  two  brass  hemi- 

Fig.  36. 
Fig.  35- 


spheres,  some  four  inches  in  diameter,  the  edges  of 
which  are  made  to  fit  tightly  together.  While  the 
hemispheres  contain  air,  they  can  be  separated  with 
ease,  since  the  outward  pressure  is  just  balanced  by  the 
inward  pressure ;  but  when  the  air  within  is  pumped 
out,  it  is  very  hard  to  pull  them  apart.  Since  it  is 
equally  difficult  to  do  this,  in  whatever  position  the 
hemispheres  are  held,  the  experiment  shows  that  the 
air  presses  in  all  directions. 

This   piece  of  apparatus   is   called   the   Magdeburg 


48 


MECHANICS. 


Fig.  37- 


hemispheres,  from  Otto  von  Guericke,  of  Magdeburg, 
by  whom  it  was  invented. 

If  a  small  bell-jar,  open  at  both  ends,  be  covered  with 
the  palm  of  the  hand,  and  the  air  be  then  exhausted 
from  it,  the  hand  will  be  held  down  with  considerable 
force  by  the  pressure  of  the  air  upon  it. 

If  a  wet  bladder  be  tied  over  the  same  bell-jar  and 
dried,  and  the  air  be  exhausted  as  before,  the  bladder 
will  burst  with  a  loud  noise. 

These  two  experiments  illustrate  the  downward  pres- 
sure of  the  air. 

In  Figure  37,  A  is  a  strong 
glass  cylinder,  open  at  both 
ends ;  B  a  piston,  working  air- 
tight within  it ;  and  C  a  brass 
plate,  covering  it  closely,  and 
having  a  hole  in  the  centre  to 
which  a  hose  may  be  screwed 
fqr  connecting  it  with  the  air- 
pump.  When  the  air  is  ex- 
hausted from  the  cylinder,  the 
piston  rises,  even  if  a  heavy 
weight  be  fastened  to  it. 

This  experiment  affords  a 
very  striking  illustration  of  the 
upward  pressure  of  the  air. 

Fig  s8  68.     Gases    have    an    Expansive 

Force.  —  If  an  india-rubber  bag,  par- 
tially filled  with  air,  be  closed  air- 
tight and  placed  under  the  receiver 
of  the  air-pump,  the  bag  fills  out  as 
shown  in  Figure  38,  when  the  air 
is  exhausted  from  the  receiver.  All 
gases  thus  tend  to  expand. 

69.   The  Air-Pump.  —  An  instru- 


MECHANICS. 


49 


t  for  removing  the  air  from  a  vessel  is  called  an 
air-pump.  One  form  of  it  is  shown  in  Figure  39.  It 
consists  of  a  cylinder,  in  which  a  piston  moves  air-tight. 
In  this  piston  is  a  valve  opening  upward.  At  the  top  of 
the  cylinder  is  another  valve  also  opening  upward.  The 
bottom  of  the  cylinder  is  connected  with  the  pump-plate 
by  a  tube.  On  this  plate  is  placed  the  vessel,  or  re- 
ceiver, from  which  the  air  is  to  be  exhausted.  As  the 

Fig.  39- 


piston  is  forced  down,  the  expansive  force  of  the  air 
below  pushes  open  the  valve  in  the  piston  to  get  into 
the  space  left  behind  it.  When  the  piston  is  drawn 
up  again,  the  expansive  force  of  the  air  above  closes  this 
valve  and  opens  the  valve  at  the  top  of  the  cylinder,  so 
that  this  air  escapes.  The  expansive  force  of  the  air  in 
the  tube  and  receiver  causes  it  to  fill  the  space  behind 
the  piston.  When  the  piston  is  again  pushed  down,  the 

4 


5O  MECHANICS. 

downward  pressure  of  the  air  outside  closes  the  valve 
at  the  top  of  the  cylinder,  while  the  expansive  force 
of  the  air  below  opens  the  valve  in  the  piston,  and 
some  of  the  air  passes  through  it.  On  drawing  up  the 
piston  again,  this  air  is  removed  as  before.  By  continu- 
ing this  process,  the  air  is  nearly  all  withdrawn  from  the 
receiver.  It  cannot  be  wholly  withdrawn,  because  as  it 
becomes  more  and  more  exhausted,  the  expansive  force 
becomes  less  and  less,  until  at  last  it  is  not  sufficient  to 
open  the  valve  in  the  piston. 

70.  A  Body  is  buoyed  up  in  the  Air.  —  If  a  hollow 
sphere  be  balanced  in  the  air  by  a  piece  of  lead,  and 
then  the  whole  apparatus  be  put  under  the  receiver  of 
an  air-pump   and  the  air  exhausted,  the   lead  will    no 
longer  balance  the  sphere.     This  shows  that  a  body  is 
buoyed  up  in  the  air  as  well  as  in  a  liquid  (61).     Bodies 
seem  to  be  lighter  in  the  air  than  in  a  vacuum  (that  is, 
a  space  from  which  the  air  has  been  exhausted],  for 
the  same  reason  that  a  body  seems  lighter  in  water  than 
in  the  air.     The  upward  pressure  of  the  air  upon  the 
bottom  of  the  body  is  somewhat  greater  than  the  down- 
ward pressure  upon  the  top  of  the  body.     A  body  in  the 
air,  then,  is  buoyed  up  by  a  force  just  equal  to  the 
weight  of  the  air  which  it  displaces.     If  a  body  weighs 
more  than  the  air  it  displaces,  it  sinks  through  the  air ; 
if  it  weighs  less  than  the  air  it  displaces,  it  rises  in  the 
air. 

71.  Balloons.  —  Balloons  rise   in 
the  air  because  they  are  Jilled  with 
some  substance  which  makes  them 
lighter  than    the  air  which   they 
displace. 

If  a  glass  bulb  and  tube  filled 
with  air  be  arranged,  as  in  Figure 
40,  with  the  end  of  the  tube  under 


MECHANICS. 


water,  and  the  bulb  be  heated  by  means  of  *.  lamp, 
the  air  in  it  expands,  and  a  part  of  it  is  driven  out  in 
bubbles  through  the  water.  This  shows  that  air  ex~ 
panels' ivhen  heated. 

Paper  balloons  are  sometimes  made  which  are  sent 
up  by  fastening  a  light  just  under  an  opening  in  the 
bottom  of  the  balloon.  The  light  heats  the  air  inside, 
and  causes  it  to  expand,  and  a  part  to  pass  out.  The 
remainder  is  then  lighter  than  the  air  displaced  by  the 
balloon,  which  consequently  rises.  Large  balloons  are 
made  of  strong  silk',  and  filled  with  some  very  light  gas, 
such  as  coal-gas.  This  makes  the  balloon  so  much 
lighter  than  the  air  it  displaces,  that  it  will  rise,  carry- 


Fig.  41. 


ing  a  car  with  two  or  three 
persons  in  it. 

72.  The  Pressure  of  the 
Atmosphere  will  sustain  a 
Column  of  Liquid  in  an  in- 
verted Vessel. — Fill  a  glass 
jar  with  water  and  invert  it 
in  a  dish  of  water,  keeping 
the  mouth  of  the  jar  all  the 
time  under  water ;  and  the 
liquid  will  not  flow  out  of 
the  jar  when  it  is  raised. 
Now  place  the  dish  and  jar 
under  the  receiver  of  an  air- 
pump,  and  exhaust  the  air ; 
and  the  water  will  flow  out 
from  the  jar.  This  shows 
that  it  is  the  pressure  of 
flic  atmosphere  on  the  sur- 
face of  the  water  in  the  dish 
which  keeps  the  water  in 
the  inverted  jar. 


52  MECHANICS. 

73.  The   Atmospheric  Pressure  will  sustain  a  Col- 
umn of  Mercury  about   30  Inches  high.  —  If  a  glass 
tube  closed  at  one  end  and  about  34  inches  long  be  filled 
with   mercury,    and   inverted  in  a  cup  of  mercury,   as 
shown  in  Figure  41,  a  part  of  the  mercury  will  run  out, 
leaving  a  column  about  30  inches  high  in  the  tube. 

74.  The  Atmospheric  Pressure  is  equal  to  about  15 
Pounds  to  the  Square  Inch.  —  If  the  tube  in  the  above 
experiment  be  one  inch  square,  it  follows,  from  the  way 
in  which  liquids  press,  that  the  downward  pressure  at 
the  bottom  of  the  tube  will  be  just  equal  to  the  down- 
ward  pressure  of  the  atmosphere  on  each  square  inch 
of  the  surface  of  the  mercury  in  the  vessel. 

If  now  we  weigh  the  mercury  in  the  tube,  we  shall 
find  that  there  are  about  15  pounds  of  it.  This  column 
of  mercury  then  exerts  a  pressure  of  15  pounds  at  the 
bottom  of  the  tube.  The  air  then  presses  with  a  weight 
of  15  pounds  upon  every  square  inch  of  surface.  We  do 
not  perceive  this  great  pressure,  because  the  air  presses 
equally  in  every  direction. 

75.  The  Atmospheric  Pressure  varies  from  Day  to 
Day.  —  If  a  glass  tube  be  filled  with  mercury,  and  then 
inverted  in  a  cup  of  mercury  and  left  standing,  and  the 
height  of  the  mercury  column  noted  from  day  to  day,  it 
will  be  found  to  vary  considerably,  being  sometimes  as 
much  as  two  inches  higher  than  at  other  times.    This  va- 
riation must  be  due  to  changes  in  the  pressure  of  the  air. 

76.  The  higher  the  Place,  the  less  the  Atmospheric 
Pressure. —  If  the  tube  just  described  be  carried  to  the 
top  of  a  mountain,  the  mercury  will  fall  considerably. 
This  shows  that  the  atmospheric  pressure  becomes  less, 
the  higher  we  go  above  the  surface  of  the  earth. 

The  atmosphere  is  a  great  ocean  of  air  which  sur- 
rounds the  earth,  and  at  the  bottom  of  which  we  live,  as 
the  fishes  live  at  the  bottom  of  the  sea.  The  changes  in 


MECHANICS. 


53 


the  height  of  the  mercury  just  described  show  that  the 

pressure  increases  with  the  depth.* 

77.    The  Barometer.  —  An  instrument  for  measuring 

the  pressure  of  the  atmosphere  is  called  a  barometer. 
One  form  of  it  is  shown  in  Figure  42.  It 
consists  of  a  cup  and  tube  filled  with  mer- 
cury, as  in  the  experiment  illustrated  by 
Figure  41.  These  are  fastened  to  a  wood- 
en frame.  At  the  upper  part  of  the  tube, 
there  is  a  scale  with  a  sliding  index,  for 
measuring  the  height  of  the  mercury.  H 
is  a  thermometer. 

The  mercury  is  often  put  into  a  leather 
bag  instead  of  an  open  cup  as  here,  since  it 
is  less  likely  to  be  spilled.  As  the  leather 
is  flexible,  the  pressure  of  the  air  is  brought 
to  bear  upon  the  mercury  through  the  bag. 

78.  Uses  of  the  Barometer.  —  When  we 
have  found  at  what  rate  the  atmospheric 
pressure  diminishes  as  we  go  up,  we  can 
readily  find  the  height  of  mountains  by 
means  of  the  barometer.  The  difference 
between  the  readings  of  the  barometer  at 
the  level  of  the  sea  and  at  the  top  of  the 
mountain,  will  show  how  much  the  pressure 
has  diminished",  and  from  this  we  can  find 
£he  height  of  the  mountain. 

The  barometer  is  also  of  considerable  use 
in  indicating  the  approach  of  storms,  espe- 
cially of  violent  winds.  It  has  been  ob- 
served that  such  storms  are  very  likely  to 
occur  immediately  after  a  sudden  diminu- 
tion of  atmospheric  pressure,  which  is  shown 

*  See,   in  the  Appendix,  the  chapter  on   The  Physics  of  the 
Atmosphere. 


54  MECHANICS. 

by  a  rapid  fall  of  the  mercury.'  On  the  other  hand,  a 
gradual  rise  of  the  mercury  usually  indicates  the  ap- 
proach of  fair  weather. 

The  mere  height  of  the  mercury  tells  us  little  about 
the  weather,  but  a  careful  study  of  its  movements  en- 
ables us  to  judge  pretty  accurately  what  changes  are 
likely  to  occur  in  the  weather. 

79.  Pumps.  —  As  water  is  somewhat  more  than  thir- 
teen times  lighter  than  mercury,  the  pressure  of  the 
atmosphere  will  sustain  a  column  of  this  liquid  about 
thirteen  times  thirty  inches  in  height,  or  considerably 
more  than  thirty  feet.  If  the  tube  is  open  at  the  top, 
it  is  necessary  to  remove  the  air  from  it,  before  the 
water  will  rise  into  it.  An  instrument  for  raising  water 
in  this  way  is  called  a  pump. 

The  common  lifting-pump  is  shown  in  Figure  43. 
It  is  really  an  air-pump.  When  the  piston  P  is  forced 
down,  the  air  below  it,  by  its  expansive  force,  opens  the 
valve  0,  through  which  it  escapes.  When  the  piston 
is  drawn  up  again,  the  valve  O  is  kept  shut  by  the 
pressure  of  the  air  above,  and  the  air  in  A  expands, 
pushes  open  the  valve  61,  and  rushes  into  the  vacuum 
above.  The  air  being  thus  partly  removed  from  A,  the 
pressure  of  the  air  upon  the  water  in  the  well  outside 
is  greater  than  that  inside  the  pipe,  and  consequently 
forces  the  water  up  the  pipe  and  through  the  open 
valve  S.  When  the  piston  is  pushed  down  again,  the 
pressure  of  the  water  in  the  cylinder  shuts  the  valve  S^ 
and  opens  the  valve  O.  The  water  thus  gets  above  the 
piston,  which,  on  going  up  again,  lifts  it  so  that  it  flows 
out  at  the  spout,  as  shown  in  the  figure. 

Figure  44  represents  the  force-pump.  In  this  pump, 
the  piston  P  is  solid.  When  it  is  drawn  up,  the  water 
below,  by  its  upward  pressure,  opens  the  valve  •S,  and 
fills  the  cylinder.  When  the  piston  is  pushed  down,  the 


MECHANICS. 


valve  61  being  shut  by  its  own  weight,  and  the  pressure 
of  the  water  upon  it,  the  water  is  forced  up  through  the 

Fig-  44- 


valve  O  into  the  pipe  D.  When  the  piston  goes  up 
again,  the  valve  O  is  closed  by  its  own  weight  and  that 
of  the  water  above,  the  valve  5  opens,  and  the  cylinder 
is  filled  as  before. 

In  Figure  45,  we  have  these  two  pumps  combined. 
The  air  is  pumped  out  through  the  valves  .5  and  O, 
and  the  water  is  forced  up  into  the  cylinder  through 
the  pipe  A  and  the  valve  S,  just  as  it  was  in  the  litting- 
pump ;  and  the  water  is  then  forced  through  the  valve 
O  and  the  pipe  Z?,  as  in  the  force-pump,  just  described. 


MECHANICS. 


In  both  these  forms  of  force-pump,  the  water  is  driven 
out  of  the  pipe  D  only  when  the  piston  is  going  down. 
It  may  be  made  to  flow  out  in  a  steady  strearn  by  adding 
an  air-chamber  above  the  valve  0,  as  shown  in  Figure 
46.  As  the  water  is  forced  into  this  chamber,  it  corn- 


Fig.  45- 


Fig.  46. 


presses  the  air,  which,  by  its  expansive  force,  exerts  a 
continuous  pressure  on  the  water,  and  drives  it  in  a 
steady  stream  up  the  pipe. 

In  tint  fire-engine,  two  force-pumps  are  usually  con- 
nected with  one  air-chamber.  The  pumps  are  so  arranged 
that  the  piston  of  one  is  going  down  while  that  of  the 
other  is  going  up,  thus  forcing  water  into  the  air-chamber 
all  the  time. 

80.  The  Siphon.  — Bend  a  tube  into  the  form  of  the  let- 
ter U,  making  one  arm  somewhat  longer  than  the  other ; 
fill  it  with  water,  and  close  both  ends  with  the  fingers ; 
then  invert  it,  and  place  the  short  end  under  the  surface 
of  water  in  a  vessel.  If  now  both  ends  be  opened,  the 
water  will  flow  out  of  the  vessel  through  the  tube.  A 
bent  tube  used  in  this  way  is  called  a  siphon. 

To  explain  the  action  of  a  siphon,  let  us  suppose  it 


MECHANICS. 


57 


Fig-  47- 


filled,   and   the    short  arm   placed    in   the  water.      The 

pressure  then  acting  on  C 
(Figure  47),  and  tending 
to  raise  the  water  in  the 
tube,  is  the  atmospheric 
pressure  less  the  weight  of 
the  column  of  water  CD. 
In  like  manner,  the  pres- 
sure on  the  end  of  the  tube 
B  is  the  atmospheric  pres- 
sure less  the  pressure  of 
the  column  of  water  A  B. 
But  as  this  latter  column 
is  longer  than  C  D,  the 
force  acting  at  B  is  less 
than  the  force  acting  at  C,  and  consequently  the  water 
will  be  driven  through  the  tube  by  a  force  equal  to  the 
difference  of  these  two  forces.  The  flow  will  therefore 
be  the  faster,  as  the  difference  of  level  between  C  and  B 
is  greater. 

Si.  Tantalus's  Cup.  —  This  is  a  glass  cup,  with  a 
siphon  tube  passing  through  the  bottom,  as  shown  in 
Figure  48.  If  water  be  poured 
into  the  cup,  it  will  rise  both  in- 
side and  outside  the  siphon  until 
it  has  reached  the  top  of  the  tube, 
when  it  will  begin  to  flow  out. 
If  the  water  runs  into  the  cup 
less  rapidly  than  the  siphon  car- 
ries it  out,  it  will  sink  in  the  cup 
until  the  shorter  arm  no  longer 
dips  into  the  liquid  and  the  flow 
from  the  siphon  ceases.  The  cup  will  then  fill  again  as 
before  ;  and  so  on. 

In  many  places  there  are  springs  which  flow  at  inter- 


Fig.  48. 


58  MECHANICS. 

vals,  like  the  siphon  in  this  experiment,  and  whose  action 
may  be  explained  in  the  same  way.  A  cavity  under 
ground  may  be  gradually  filled  with  water  by  springs, 
and  then  emptied  through  an  opening  which  forms  a 
natural  siphon.  In  some  cases  of  this  kind  the  flow 
stops  and  begins  again  several  times  in  an  hour. 

82.  The  Air- Gun  and  the  Condenser.  —  The  expan- 
sive force  of  gases  increases  'when  they  are  compressed. 
This  is  illustrated  by  the  air-gun,  which  consists  of  a 
tube,    connected  by  a  stop-cock   with  a  small   air-tight 
vessel  of  very  great  strength.     If  a  large  amount  of  air 
be  forced   into   this  vessel,   and   the   stop-cock  be  then 
opened,  the  expansive  force  of  the  gas  will  drive  a  bul- 
let from  the  tube,  as  if  it  were  fired  from  a  musket. 

The  firing  of  a  musket  is,  in  fact,  another  illustration 
of  the  very  same  kind.  When  the  gunpowder  is  set  on 
fire,  it  forms  an  immense  amount  of  gas,  which,  being 
confined  in  a  small  space,  has  a  great  expansive  force, 
and  therefore  exerts  a  great  pressure  upon  the  bullet. 

An  instrument  used  for  compressing  air  is  called  a 
condenser.  It  consists  of  a  strong  cylinder,  with  a  pis- 
ton and  valves,  arranged  precisely  as  in  the  force-pump 
in  Figure  44.  It  works,  too,  in  the  same  way  as  the 
force-pump ;  the  air  rushing  in  through  the  valve  S 
when  the  piston  is  raised,  and  being  driven  out  through 
the  valve  O  when  the  piston  is  pushed  down.  The  ves- 
sel into  which  the  air  is  to  be  forced  is  screwed  to  D. 

83.  Mariotte's  Law.  —  The  bulk  of  a  gas  becomes 
less  just  in  proportion  as  the  pressure  upon  it  becomes 
greater ;  or,  in  other  words,  the  volume  of  a  gas  is  in- 
versely as  the  pressure  which  it  bears. 

The  elasticity  of  a  gas  becomes  greater  just  in  propor- 
tion as  its  bulk  becomes  less,  or  as  the  pressure  upon  it 
becomes  greater ;  or,  in  other  words,  the  elasticity  of  a 
gas  is  inversely  as  its  volume,  and  directly  as  the  pres- 


MECHANICS.  59 

sure  which  it  bears.     These  facts  concerning  gases  are 
known  as  Mariettas  law,  from  their  discoverer. 

84.  The  Spirit  Level.  —  The  spirit  level  (Figure  49) 
consists  of  a  closed  glass  tube,  A  B,  very  slightly  curved 
on  the  upper  side.  It  is  filled  with  spirit,  with  the  ex- 

Fig.  49- 


D 


ception  of  a  bubble  of  air  which  tends  to  rise  to  the 
highest  part  of  the  tube.  It  is  set  in  a  case  C  *D ;  and 
when  it  is  placed  on  a  perfectly  level  surface,  the  bubble 
is  exactly  in  the  middle  of  the  tube,  as  in  the  figure. 


SUMMARY. 

Gases  have  weight,  and,  like  liquids,  press  upward, 
downward,  and  sideways.  (66,  67.) 

Gases  are  acted  upon  by  an  expansive  force,  which  is 
increased  by  heat  and  by  pressure.  (68,  71,  82.) 

Bodies  are  buoyed  up  in  air  by  a  force  equal  to  the 
weight  of  the  air  which  they  displace.  (70.) 

The  atmospheric  pressure  balances  a  column  of  mer- 
cury about  thirty  inches  high,  and  is  equal  to  about 
fifteen  pounds  to  the  square  inch.  (73,  74.) 

This  pressure  varies  from  day  to  day,  and  becomes 
less  as  the  height  of  the  place  increases.  (75,  76.) 

The  barometer  is  an  instrument  for  measuring  the  at- 
mospheric pressure.  (77?  7^0 

The  action  of  pumps,  and  of  the  siphon,  is  to  be  ex- 
plained by  the  pressure  of  the  atmosphere.  (79,  80.) 

The  bulk,  or  volume,  of  a  gas  is  in  the  inverse 
ratio  of  the  pressure  which  it  bears.  The  elasticity 
of  a  gas  is  in  the  inverse  ratio  of  its  volume,  or  the 
direct  ratio  of  the  pressure  it  bears.  (83.) 


60  MECHANICS. 

MOTION. 
FIRST  LAW  OF  MOTION. 

85.  Inertia.  —  We  know  that  a  stone,  or  other  body, 
when  at  rest,  will  not  begin  to  move  of  itself,  but  only 
on  the  application  of  some  force ;   and  that,  when  any 
body,  as  a  ball,  is  in  motion,  it  requires  some  force  to 
stop  it. 

The  inability  of  a  body,  whether  at  rest  or  in  mo- 
tion, to  change  its  state,  is  often  called  inertia. 

86.  A  moving  Body,  when  left  to  itself,  will  always 
move  in  a  straight  Line  and  at  the  same  Rate.  — 
Mathematicians,  from  the  study  of  certain  motions,  have 
come  to  the  conclusion  that  a  moving  body,  when  left 
to  itself,  will  always  move  in  a  straight  line  and  at  the 
same  rate.     This  is  the  first  law  of  motion. 

87.  An  unbalanced  Force  must  act  upon  a  Body  in 
order  to  put  it  in  Motion,  or  to  change  the  Direction 
or  the  Rate  of  its  Motion.  —  A  ball,  held  in  the  hand, 
remains  at  rest,  because  the  downward  pull  of  gravity 
upon  the  ball  is  just  balanced  by  the  resistance  offered 
by  the  hand.     If  the  hand  is  removed  so  that  the  force 
of  gravity  is  unbalanced,  then  the  ball  begins  to  move. 
If  we  push  with  the  hands  against  the   opposite  sides 
of  a  book,  the  book  will  remain  at  rest  as  long  as  the 
push  of  one  hand  is  just  balanced  by  that  of  the  other. 
Take  away  one  hand,  so  that  there  shall  be  nothing  to 
balance  the  push  of  the  other,  and  the  book  begins  to 
move.     So,  in  every  case,  a  body  begins  to  move  only 
when  an  unbalanced  force  acts  upon  it. 

And  when  a  body  is  once  in  motion,  it  changes  the 
direction  and  rate  of  its  motion  only  when   an  un- 


MECHANICS.  6 1 

balanced  force  is  acting  upon  it.  When  a  body  is 
once  in  motion,  it  is  just  as  natural  for  it  to  move  on 
in  a  straight  line,  with  uniform  speed,  as  it  is  for  it  to 
remain  at  rest  when  once  it  is  at  rest. 

88.  The  Effect  of  a  Force  acting  for  a  Moment  only. 
—  When  a  body  is  acted  upon  by  a  force  only -for  an 
instant,  as  when  a  ball  is  struck  with  a  bat,  or  a  bullet 
is  fired  from  a  gun,  it  has  its  greatest  speed  at  first, 
and  its  motion  is  gradttally  wasted  by  the  resistance 
it  meets  in  passing  through  the  air  or  over  the  earth. 

89.  The  Effect  of  a  Force  acting  continuously. — 
When  a  body  is  acted  upon  continuously  by  a  force,  as 
in  the  case  of  a  railway  train,  "or  a  steamboat,  the  mo- 
tion, slow  at  first,  gradually  increases  till  it  reaches 
a  certain  point,  when  the  speed  remains  unchanged 
so  long  as  the  moving  force  is  unchanged.     When  the 
moving   force    is   increased,    the    speed   increases;    and, 
when  it  is  diminished,  the  speed  diminishes. 

90.  The  Resistance  a  moving  Body  meets  increases 
as   the    Square   of  its    Velocity.  —  The    steamboat,   in 
moving,  has  to  push   aside  a  certain  amount  of  water 
in  a  second,  and  this  is  the  chief  resistance  it  meets. 
Now,   as  the  speed  of  the  boat  increases,   more  water 
must  be  pushed  aside  in  a  second,  and  each  particle  of 
water  must  be  moved  aside  more  quickly.     Hence,  the 
faster   it   moves,    the   greater   the    resistance.      Suppose 
die  speed  of  the  boat  to  be  doubled,  twice  as  many  par- 
ticles of  water  must  be  pushed  aside   in  a  second,  and 
each   particle  must  be  pushed   aside    in  half  the  time. 
Hence,  the  resistance  becomes  fourfold  when  the  velocity 
is  doubled.    The  resistance,  then,  increases  as  the  square 
yf  the  velocity. 

91.  A  moving  Body  may  be  in  Equilibrium.  —  A 
body  at  rest  is  in  equilibrium,  because  the  forces  acting 
upon  it  are  balanced.     When  a  train  of  cars  is  starting, 


62  MECHANICS. 

the  force  of  the  steam  is  not  wholly  balanced  by  the  re- 
sistance ;  hence  it  imparts  motion  to  the  train.  But  as 
the  speed  of  the  train  increases,  the  resistance  also  in- 
creases, until  it  finally  equals  the  force  of  the  steam. 
All  the  force  of  the  steam  is  now  used  in  balancing  the 
resistance,  and  the  speed  no  longer  changes.  Since  the 
two  forces  acting  upon  the  moving  body  balance  each 
other,  it  must  be  in  equilibrium.  Every  body  moving 
in  a  straight  line,  and  with  uniform  speed,  is  in 
equilibrium. 

SECOND  LAW  OF  MOTION. 

92.  A  Force  has  the  same  Effect  in  producing  Mo- 
tion, whether  it  acts  upon  a  Body  at  Rest  or  in  Motion, 
and  whether  it  acts  alone  or  with  other  Forces.  — 
When  a  ball  is  thrown  horizontally,  two  forces  act 
upon  it,  one  to  throw  it  forward  in  a  straight  line, 
and  the  other  to  draw  it  to  the  earth  in  a  straight  line ; 
and  it  is  found  that  it  is  drawn  just  as  far  towards 
the  earth  in  a  given  time  as  a  ball  that  is  let  fall 
from  a  state  of  rest.  The  same  is  true,  in  whatever 
direction  the  ball  may  be  thrown. 

For  example,  if  the  force  used  would  send  the  ball 
forward  30  feet  in  a  second,  and  if  gravity  will  pull  it 
from  a  state  of  rest  16  feet  towards  the  earth  in  the 
same  time,  the  ball  at  the  end  of  the  second  will  be  just 
16  feet  below  the  point  it  would  >  have  reached  had 
not  the  force,  of  gravity  acted  upon  it.  So,  were  a 
ball  thrown  directly  upward  with  a  velocity  of  100  feet 
a  second,  at  the  end  of  the  second  it  would  be  only  84 
feet  high  ;  that  is,  16  feet  below  the  point  it  would  have 
reached  had  not  the  force  of  gravity  acted  upon  it.  If 
it  were  thrown  directly  downward  from  the  top  of  a  high 
tower  with  the  same  velocity,  it  would  be  at  the  end  of 


MECHANICS.  63 

a  second  116  feet  below  the  top  of  the  tower ;  that  is,  16 
feet  below  the  point  it  would  have  reached  had  not 
gravity  acted  upon  it.  Now  16  feet  is  just  the  distance 
in  each  case  that  gravity  would  have  pulled  the  ball  in 
a  second  from  a  state  of  rest. 

Again,  if  the  current  in  a  stream  is  strong  enough  to 
carry  a  boat  down  stream  one  mile  in  an  hour,  and 
if  a  person  attempts  to  row  the  boat  directly  across 
the  stream  at  a  rate  which  would  take  him  across  in 
an  hour,  at  the  end  of  the  hour  the  boat  will  be  at 
the  opposite  bank  just  a  mile  down  stream. 

93.  A  Body  thrown  horizontally  or  Fig.  50. 
obliquely,  when  acted  upon  by  Grav- 
ity, describes  a  curved  Path.  — When 

both  the  forces  acting  upon  the  body 
are  instantaneous,  it  moves  in  a 
straight  line;  when  one  is  instan- 
taneous, and  the  other  continuous,  as 
in  the  case  of  gravity  acting  on  a  ball 
thrown  horizontally  or  obliquely,  the 
path  is  curved.  Hence  a  cannon- 
ball  describes  a  curved  path  ;  if  fired 
at  a  distant  object,  it  must  be  aimed 
above  it. 

94.  All  Bodies  would  fall  at  the 
same  Rate,  were  it  not  for  the  Resist- 
ance of  the  Air.  —  As  we  see  bodies, 
light   and   heavy,  falling  through   the 
air,  we  come  to  think  that  the  force 
of  gravity  causes  heavy  bodies  to  fall 
more  rapidly  than  light  ones  ;  but  if  we 
place  a  coin  and  a  feather  in  a  long 
glass  tube   and   exhaust   the  air  com- 
pletely   (Figure   50),    the   two   bodies 

will  fall  through  the  tube  in  the  same  time.     It  must, 


64  MECHANICS. 

then,  be  the  resistance  of  the  air  which  causes  a  lighter 
body  to  fall  more  slowly  through  the  atmosphere  than  a 
heavy  one  does. 

When  the  force  of  gravity  is  unimpeded  in  its  action, 
it  will  cause  every  body,  whatever  may  be  its  size, 
shape,  or  density,  to  fall  with  exactly  the  same  speed. 

95.  When  a  Body   is   moving  directly  downward, 
Gravity  increases  its  Velocity  at  the  Rate  of  32  Feet 
a  Second.  —  A  body  falls  16  feet  the  first  second,  and 
acquires  a  velocity  of  32  feet  during  the  time.     As  grav- 
ity has  the  same  effect  upon  a  moving  body  as  upon  one 
at  rest,  a  falling  body  will  gain  in  velocity  32  feet  each 
second.   When  therefore  a  body  is  moving  directly  down- 
ward, gravity  increases  its  velocity  at  the  rate  of  32  feet 
a  second. 

96.  How  to  find  the  Distance  a  Body  falls  in  a  given 
Time.  —  The  distance  a  body  falls  the  first  second,  or  16 
feet,  is  exactly  the  mean  between  o,  its  velocity  at  start- 
ing, and  32,  its  velocity  at  the  end  of  the  second.     As 
it  would  gain  a  velocity  of  32  feet  during  the  next  second, 
it  would  have  a  velocity  of  64  feet  at  the  end  of  that 
second.    The  velocity  it  has  already  acquired  would  cause 
it  to  fall  32  feet  the  second  second,  and  the  force  of  gravity 
acting  upon  it  during  that  time  would  cause  it  to  fall  16 
feet  more  ;  hence  it  would  fall  48  feet  during  the  second 
second.     It  will  be  noticed  that  48  is  just  the  mean  of 
32,  its  velocity  at  the  beginning  of  the  second,  and  64,  its 
velocity  at  the  end  of  the  second. 

During  the  first  two  seconds,  the  body  would  fall  48  -|- 
16  ==.  64  feet.  This  is  just  twice  the  mean  of  o  and  64. 
Hence,  to  find  the  distance  that  any  body  would  fall 
when  acted  upon  by  gravity  alone  during  any  number 
of  seconds,  find  its  mean  velocity  during  the  time,  and 
multiply  it  by  the  number  of  seconds. 

To  find  the  velocity  of  a  falling  body  at  the  end  of 


MECHANICS.  65 

any  second,  multiply  32  feet  by  the  number  of  seconds 
it  has  been  falling. 

97.  When  a  Body  is  moving  directly  upward,  Grav- 
ity retards  its  Velocity  at  the  Rate  of  32  Feet  a  Second. 
—  We  know  that  gravity  has  the  same  effect  on  a  body 
in  motion  as  on  one  at  rest  (92).     Since,  then,  it  causes 
a  body  in  falling  from  a  state  of  rest  to  acquire  a  velocity 
of  32  feet  a  second,  it  must,  in  the  case  of  a  body  mov- 
ing directly  upward,  diminish  its  velocity  at  the  rate 
of  32  feet  a  second.     And  it  must  also  cause  it  to  rise 
each  second  16  feet  less  than  if  it  were  not  acting 
upon  it. 

98.  How  to  Jind  the  Distance  a  Body,  when  thrown 
upward,  will  rise  in  a  given  Time.  —  To  find  this  dis- 
tance, take  the  mean  velocity  of  the  body  during  the 
time,  and  multiply  it  by  the  number  of  seconds.     To 
find  the  velocity  at  any  particular  second,  multiply  the 
number  of  seconds  the  body   has  been    rising  by  32, 
and  subtract  this  from  the  velocity  the  body  has  at 
starting. 

THIRD  LAW  OF  MOTION. 

99.  Momentum.  —  The  product  of  the  velocity  of  a 
body  multiplied  by  its  mass  is  called  its  momentum. 
By  the  mass  of  a  body,  we  mean  its  quantity  of  mat- 
ter.    The    same    force   acting    upon   bodies   containing 
different  quantities    of  matter   does   not   give    each   the 
same  velocity ;  but  it  does  give  each  the  same  momen- 
tum, or  quantity  of  motion;  that  is,  if  the  quantity  of 
matter  in  each  be  multiplied  by  its  velocity,  the  prod- 
ucts will  all  be   equal. 

100.  A    moving   Body    cannot    impart    Motion    to 
another  Body  without  itself  losing  the  same  Quan- 
tity of  Motion.  —  This  third  law  of  motion  results  from 

5 


66  MECHANICS. 

the  fact  that  a  moving  body  is  unable  of  itself  either  to 
increase  or  to  lessen  its  quantity  of  motion.  On  meeting 
another  body,  it  may  impart  some  of  its  own  motion  to 
it ;  but  it  cannot  give  motion  to  this  body,  and  at  the 
same  time  retain  all  its  own  motion. 

This  is  often  called  the  law  of  action  and  reaction, 
and  stated  thus :  action  and  reaction  are  always  equal 
and  in  opposite  directions.  When  any  force  acts  in  op- 
posite directions,  it  is  usually  said  to  act  in  one  direction, 
and  react  in  the  opposite.  Thus  in  firing  a  cannon, 
the  expansive  force  of  the  gases  set  free  by  the  burning 
powder  acts  equally  in  all  directions.  It  acts  upon  the 
sides  with  equal  and  opposite  forces  which  neutralize 
each  other  unless  the  cannon  bursts.  It  also  acts  toward 
the  muzzle  and  breech  with  equal  forces,  which  produce 
equal  effects,  one  upon  the  ball  and  the  other  on  the  can- 
non, causing  the  recoil.  The  ball  and  the  cannon  both 
have  the  same  momentum ;  but  the  ball,  since  it  has  a 
much  less  mass,  gets  a  much  greater  velocity.  This 
expansive  force  is  said  to  act  upon  the  ball  and  to  react 
upon  the  gun.  So,  too,  in  walking,  we  are  said  to 
react  upon  the  earth.  The  truth  is,  that  the  bent  leg 
acts  like  a  bent  spring  between  our  bodies  and  the 
earth ;  and  when  the  spring  straightens,  it  pushes  us 
away  from  the  earth  and  the  earth  away  from  us ;  the 
earth  being  moved  as  much  less  than  our  bodies  as  its 
mass  is  greater. 

101.  It  requires  Time  to  impart  Motion  to  a  Body 
as  a  Whole.  —  The  forces  which  impart  motion  to  a 
body  often  act  directly  upon  only  a  few  of  its  particles. 
When  a  ball  is  struck  by  a  bat,  only  a  small  part  of  it 
receives  the  blow,  and  when  a  bullet  is  shot  from  a  gun, 
the  gases  (46)  act  only  upon  one-half  of  it.  In  such 
cases,  it  is  clear  that  the  motion  must  be  transmitted 
from  particle  to  particle;  and  this  transmission  of 


MECHANICS.  67 

motion  from  particle  to  particle  requires  time,  ab- 
though  this  time  may  be  exceedingly  short.  If  the 
force  acts  so  suddenly  that  there  is  not  time  enough  for 
this  transmission,  the  part  acted  upon  is  flattened  or 
chipped  off.  Thus  a  musket-ball  may  be  fired  through 
a  window-pane,  making  a  clear  round  hole  without 
cracking  the  glass.  If  the  ball  had  been  thrown  by 
the  hand,  the  whole  pane  would  have  been  shattered. 
In  the  first  case,  the  speed  of  the  ball  was  so  great  that 
the  particles  in  front  of  it  had  not  time  to  transmit 
their  motion  to  those  about  them ;  hence  they  moved 
on  alone,  leaving  the  others  at  rest.  If  the  pane  had 
been  suspended  by  a  fine  thread,  the  ball  would  have 
passed  through  it  in  the  same  way,  without  breaking 
the  thread,  or  causing  the  pane  to  swing  in  the  least. 
So  a  door  half  open  may  be  pierced  by  a  cannon-ball 
without  being  shut.  The  end  of  a  musket  in  a  soldier's 
hand  has  been  known  to  be  carried  away  by  a  cannon- 
ball  without  his  being  aware  of  it.  A  tallow  candle 
may  be  fired  through  a  board,  since  it  gets  through  it 
before  the  parts  of  the  tallow  have  time  to  yield.  In 
this  way,  a  soft  missile  may  hit  as  hard  as  lead,  if  fired 
with  sufficient  speed. 

\Ve  see,  then,  that  when  a  moving  body  meets  with 
another  it  seldom  expends  all  its  power  in  imparting 
motion  to  that  body  as  a  whole,  but  also  pierces  it  more 
or  less.  The  .power  of  a  body  to  pierce  another  in- 
creases as  the  square  root  of  its  velocity;  that  is,  if  a 
body  is  to  pierce  another  twice  as  far,  it  must  have  four 
times  the  velocity ;  if  three  times  as  far,  nine  times  the 
velocity  ;  and  so  on. 

102.  Reflected  Motion.  —  When  an  elastic  ball  is 
thrown  against  the  floor,  it  rebounds.  If  it  is  thrown 
directly  downward,  it  retraces  its  path  in  its  rebound. 
If  it  is  thrown  obliquely,  it  rebounds  obliquely  in  an 


68  MECHANICS. 

opposite  direction.     In  Figure  51,  if  the  ball  is  tnrown 
Fig.  5i.  in  the  direction  af,  it  will  rebound 

in  the  direction^*  b.  If  the  line  e  j 
be  drawn  at  right  angles  to  the  sur- 
face,  the  angle  formed  by  the  two 
lines  #  j^and  e  f\§  called  the  angle 
of'  incidence,  and  is  always  equal 
to  the  angle  formed  by  the  two  lines  b  f  and  e  f.  This 
last  angle  is  called  the  angle  of  reflection.  In  reflected 
motion,  the  angle  of  incidence  always  equals  the  angle 
of  reflection. 


SUMMARY. 

The  inability  of  a  body,  whether  at  rest  or  in  motion, 
to  change  its  state,  is  called  inertia.  (85.) 

A  moving  body,  when  left  to  itself,  will  always  move 
in  a  straight  hne  and  at  the  same  rate.  (86.) 

An  unbalanced  force  must  act  upon  a  body  in  order 
to  put  it  in  motion,  or  to  change  the  direction  or  rate  of 
its  motion.  (87.) 

When  a  force  acts  upon  a  body  for  a  moment  only, 
the  motion  which  it  imparts  is  gradually  wasted  away, 
owing  to  the  resistance  which  the  body  meets.  (88.) 

The  resistance  which  a  moving  body  meets  increases 
as  the  square  of  the  velocity  of  its  motion.  (90.) 

.A  body  moving  in  a  straight  line  and  with  uniform 
velocity  is  in  equilibrium.     (91.) 

An  unbalanced  force  has  the  same  effect,  whether  it 
act  upon  a  body  at  rest  or  in  motion,  and  whether 
it  act  alone  or  with  other  forces.  (92.) 

A  body  thrown  horizontally  or  obliquely,  when  acted 
upon  by  gravity,  is  made  to  move  in  a  curved'  path. 
(93-) 


MECHANICS.  69 

Were  it  not  for  the  air,  a  light  body  would  fall  as  fast 
as  a  heavy  one.  (94.) 

Gravity  acting  alone  causes  a  body  to  fall  from  a  state 
of  rest  about  16  feet  in  a  second.  When  a  body  is  mov- 
ing directly  downward,  gravity  increases  its  velocity  at 
the  rate  of  32  feet  a  second.  When  a  body  is  moving 
directly  upward,  gravity  retards  its  velocity  at  the  rate 
of  32  feet  a  second.  (95,  97.) 

The  same  force  always  gives  to  a  body  the  same 
quantity  of  motion,  or  momentum.  The  momentum  of 
a  body  is  found  by  multiplying  its  weight  by  its  velocity. 

(99-) 

A.    moving  body  cannot  impart  motion   to  another 

body  without  itself  losing  the  same  quantity  of  motion. 

(.00.) 

When  the  same  force  acts  in  opposite  directions,  it 
is  often  said  to  act  in  one  direction  and  to  react  in  the 
opposite.  (100.) 

It  takes  time  to  give  motion  to  a  body  as  a  whole. 

(101.) 

In  reflected  motion,  the  angle  of  incidence  equals  the 
angle  of  reflection.  (102.) 

For  Problems  under  the  Laws  of  Motion,  see  Appendix. 


THE  PENDULUM. 

103.  A  Pendulum  is  a  heavy  Body  hung  from  a 
fxed  Point  by  means  of  a  Cord  or  Rod.  —  When  the 
centre  of  gravity  of  the  body  is  directly  under  the  point 
of  support,  the  body  remains  at  rest ;  but  if  the  body  be 
drawn  out  of  this  position  and  let  go,  it  will  fall  towards 
a  vertical  line  passing  through  the  point  of  support ;  and 
when  it  has  reached  this  line,  it  will,  owing  to  its  inertia, 
pass  beyond  it.  On  coming  to  rest,  it  again  falls  toward 


MECHANICS. 


Fig.  52. 


this  vertical   line   and   again   passes  beyond,    and   thus 
continues  to  swing  from  side  to  side. 

104.  First   Law   of  the    Vibration 
of  the    Pendulum.  —  Suppose    d,    in 
Figure  52,  to  be  a  leaden  ball  hanging 
by  a  fine  silk  thread.     Pull   it  to  one 
side  so  that  it  shall  swing  through  a 
very  short  arc,  and  count  the  number 
of  its  vibrations   in   a   minute.      Now 
bring  it  to  rest  again,  and  draw  it  to 
one  side  so  that  it  shall  swing  through 
a  little  longer  arc,  and  again  count  its 
vibrations   in  a  minute.     Again  bring 
the  ball  to  rest,  then  cause  it  to  swing 
through  an  arc  yet  longer,  and  count 
the   vibrations    in    a    minute.     In    all 
three  cases,  the  number   of  vibrations 
in  a  minute  will  be  equal. 
By  a  vibration  is  meant  the  whole  of  the  pendulum's 
movement  in  one  direction.      The  arc  through  which 
the  pendulum  swings   is   called   the   amplitude  of  its 
vibration. 

When  the  length  of  the  pendulum  remains  the  same, 
the  pendulum  always  vibrates  in  nearly  the  same  time, 
whatever  be  the  amplitude  of  the  vibration.* 

This  singular  property  of  the  pendulum  is  called  isoch- 
ronism,  from  two  Greek  words,  signifying  equal  times, 
and  the  vibrations  of  the  pendulum  are  said  to  be 
isochronous.  ' 

105.  The  Second  Law  of  the  Vibration  of  the  Pen- 
dulum.  —  Let  d  and  c,  in  Figure  52,  be  two  pendulums 
exactly  alike,  except  that  the  ball  of  one  is  lead,  and  of 
the  other  ivory.  It  will  be  found  that,  making  allowance 


*  If  the  amplitude  does  not  exceed  3°,  the  time  of  vibration 
will  always  be  exactly  the  same. 


MECHANICS.  71 

for  the  resistance  of  the  air,  each  performs  the  same 
number  of  vibrations  in  the  same  time.  For  pendulums 
of  the  same  length,  the  time  of  the  vibration*  is  the 
same,  whatever  the  pendulum  may  be  made  of. 

106.  Third  Law  of  the   Vibration  of  the  Pendulum. 

—  Let  b,  in  Figure  52,  be   a  pendulum   one-fourth  the 
length  of  c,  and   a  another,  one-ninth  the  length  of  c. 
It  will  be  found  that  b  vibrates  twice  as  fast  as  c,  and  a 
three  times  as  fast  as  c.     This  shows  that,  for  pendu- 
lums of  unequal  length,  the  time  of  the  vibration  is 
proportional  to  the  square  root  of  the  length  j  that  is, 
the  lengths  of  the  pendulum  being  made  4,  9,  and  16 
times  greater,  the  times  of  vibration  will  be  only  2,  3, 
and  4  times  longer. 

107.  Fourth  Law  of  the  Vibration  of  the  Pendulum. 

—  It  is  found  that  when  a  pendulum  of  a  given  length 
is  placed  on  different  parts  of  the  earth's  surface,  the 
time  of  the  vibrations  is  not  always  the  same.     Towards 
the  poles  it  is  found  to  vibrate  more  rapidly  than  at  the 
equator.      Mathematicians  have   shown  that  this  is  be- 
cause the  force  of  gravity  is  stronger  at  the  poles.     They 
have  shown  that,  in  different  parts  of  the  earth,  the 
time  of  vibration  for  pendulums  of  the  same  length  is 
in  the  inverse  ratio  of  the  square  root  of  the  intensity 
of  gravity;  that  is,  if  the  intensity  of  gravity  were  four 
times  as  great  in  one  place  as  in  another,  the  time  of 
vibration   for   a   pendulum   of   the    same    length   would 
be  half  as  great,  and  so  on. 

108.  The  Use  of  the  Pendulum  for  Measuring  Time. 

—  The    most    important    use    of    the    pendulum    is   for 
measuring  time.     The  common  clock  is  merely  a  con- 
trivance for  recording  the  beats  of  the  pendulum,  and 
keeping  up  its  motion.     The  essential  parts  of  such  a 
clock  are  shown  in  Figure  53.     The  toothed  wheel  R, 
called  the  scape-wheel,  is  turned  by  a  weight  or  spring, 


MECHANICS. 


F»g-  S3-  and    its    motion   is    regulated    by    the 

escapement  n  m,  which  swings  on 
the  axis  o;  the  vibrations  of  the  pen- 
dulum being  communicated  to  it  by 
means  of  the  forked  arm  a  b.  When 
the  pendulum  is  at  rest,  one  of  the  teeth 
of  the  scape-wheel  rests  upon  the  upper 
side  of  the  hook  m,  and  the  clock  does 
not  go.  If  now  the  pendulum  be  set  in 
motion,  so  that  the  hook  m  is  moved 
from  the  wheel,  the  tooth  which  rested 
on  it  is  set  free,  and  the  wheel  begins  to 
turn  ;  but  it  is  soon  stopped  by  the  hook 
72,  which  moves  up  to  the  wheel  as  m 
moves  away  from  it,  and  catches  on  its 
under  side  the  tooth  next  below.  As 
the  pendulum  swings  back,  the  hook  n 
moves  away,  the  wheel  again  begins  to 
turn,  but  is  stopped  again  on  the  oppo- 
site side  by  the  hook  m,  which  catches 
the  tooth  next  to  the  one  it  held  before ; 
and  thus  each  vibration  of  the  pendulum 
allows  the  scape-wheel  to  move  forward  through  a  space 
equal  to  one-half  of  one  of  its  teeth.  If,  then,  the  wheel 
has  thirty  teeth,  it  will  turn  round  once  in  sixty  beats  of 
the  pendulum.  Upon  the  axis  of  this  wheel  the  second- 
hand of  the  clock  is  placed.  It  is  connected  with  another 
wheel,  which  takes  sixty  times  as  long  to  revolve,  and 
which  carries  the  minute-hand;  and  this  latter  wheel 
is  connected  with  another,  which  turns  in  twelve  times 
the  period,  and  carries  the  hour-hand.  Thus  the  second- 
hand registers  the  pendulum-beats  up  to  sixty,  or  one 
minute  ;  the  minute-hand  registers  the  revolutions  of  the 
second-hand  up  to  sixty,  or  one  hour ;  and  the  hour-hand 
those  of  the  minute-hand  up  to  twelve,  or  half  a  day. 


MECHANICS.  73 

Were  it  not  for  the  pendulum  and  escapement,  these 
wheels  would  be  whirled  round  very  fast  by  the  action 
of  the  weight  or  spring,  and  the  clock  would  soon  run 
down.  On  the  other  hand,  were  there  not  some  means 
of  keeping  up  the  motion  of  the  pendulum,  it  would  soon 
be  brought  to  rest  by  the  resistance  of  the  air  and  the 
friction  at  the  point  of  suspension.  Its  motion  is  kept 
up  by  means  of  the  escapement,  which  is  so  constructed 
as  to  give  it  a  slight  push  at  each  vibration.  The  ends 
of  the  two  hooks  have  inclined  surfaces  against  which 
each  tooth  of  the  wheel,  as  it  leaves  them,  presses  with 
considerable  force,  so  as  to  throw  the  escapement  for- 
ward a  little.  This  impulse  is  communicated,  through 
the  axis  o,  and  the  arm  a  b,  to  the  pendulum. 


SUMMARY. 

A  pendulum  is  a  .heavy  body  hung  from  a  fixed  point 
by  means  of  a  cord  or  rod.  (103.) 

The  laws  of  the  pendulum  are  four :  — 

\st,  When  the  length  of  the  pendulum  remains  the 
same,  the  pendulum  vibrates  in  nearly  the  same  time, 
whatever  be  the  amplitude  of  the  vibration.  (104.) 

id,  For  pendulums  of  the  same  length,  the  time  of 
the  vibrations  is  the  same,  whatever  the  pendulum  may 
be  made  of.  (105.) 

^d,  For  pendulums  of  different  lengths,  at  the  same 
place,  the  time  of  the  vibrations  is  proportional  to  the 
square  root  of  the  lengths.  (106.) 

\th,  In  different  parts  of  the  earth,  the  time  of  the 
vibrations  for  pendulums  of  the  same  length  is  in 
the  inverse  ratio  of  \the  square  root  of  the  intensity  of 
gravity.  (107.) 

The  pendulum  is  used  for  measuring  time.     (108.) 


74  MECHANICS. 

MACHINES. 
THE  LEVER. 

109.    There  are  three  Kinds  of  Lever.  —  When  a 
workman  wishes  to  raise  a  large   stone,  he  places  an 
R  iron  bar  under  it,  as  in  Figure  54, 

»      with  a  block  under  the  bar  near  the 
p   stone,  and  then  presses  down  upon 
the  other  end  of  the  bar ;  or  else  he 
places  the  end  of  the  bar  under  the 
stone,  as  in  Figure  55,  so  that  one  end  of  it  rests  upon 
the  ground,  and  then  lifts  upon  the  other  end.     The  bar 
Fig.  55.  thus    used    constitutes    one   of   the 

simple  machines.  It  is  called  the 
lever.  ,  The  mass  to  be  raised  is 
called  the  weight.  The  moving- 
force  applied  at  the  other  end  of 
the  bar  is  called  the  power;  and  the  point  on  which  the 
bar  rests  is  called  the  fulcrum.  The  parts  between  the 
fulcrum  and  the  points  where  the  power  and  weight 
act  are  the  arms  of  the  lever.  In  the  first  case,  the 
fulcrum  was  between  the  weight  and  the  power ;  in  the 
second  case,  the  weight  was  between  the  fulcrum  and 
Fig  S6  the  power.  In  the  fishing-rod 

,  (Figure   56),  one   hand,  f,  is  the 

fulcrum ;  the  other  hand,  P,  is 
the  power ;  and  the  fish  is  the 
weight.  Here  the  power  is  applied  between  the  ful- 
crum and  the  weight. 

There  are,  then,  three  kinds  of  lever :  — 
(i)  That  with  the  fulcrum  between  the  weight  and 
power. 


MECHANICS.  75 

(2)  That  'with  the  -weight  between  the  fulcrum  and 
power. 

(3)  That  with  the  power  between  the  fulcrum  and 
the  weight. 

These  three  kinds  of  lever  ate  shown  in  Figure  57» 

Fig.  57- 


1 10.  The  Law  of  the  Lever.  —  In  the  lever  of  the 
first  kind,  if  the  fulcrum  is  just  half-way  between  the 
weight  and  power,  then  the  weight  and  power  will  move 
through  equal  distances.  In  this  case,  the  weight  and 
power  must  be  equal  in  order  to  balance  each  other,  or 
to  be  in  equilibrium.  If  the  power  were  twice  as  far 
from  the  fulcrum  as  the  weight,  then  the  weight  would 
move  through  only  half  the  distance  that  the  power  does, 
and  in  this  case  the  power  need  be  only  half  the  weight 
in  order  to  balance  it. 

Thus  we  see  that,  in  the  case  of  the  lever,  the  weight 
and  power  will  balance  each  other  when  the  power, 
multiplied  by  the  distance  through  which  it  moves, 
equals  the  weight  multiplied  by  the  distance  through 
which  it  moves;  that  is,  if  the  fulcrum  of  a  lever  be 
so  placed  that  one  end  of  the  lever  will  move  through  a 
thousand  inches  while  the  other  end  moves  one  inch, 
then  a  power  of  one  pound  on  the  former  will  balance 
a  weight  of  a  thousand  pounds  on  the  latter.  ^>s 

in.   The  Law  of  Machines  in  General.  —  The  same 


/ 


76  MECHANICS. 

is  found  to  be  true  in  the  case  of  every  machine,  how- 
ever complicated ;  namely,  that  the  power  and  'weight 
'will  balance  each  other  when  the  power,  multiplied  by 
the  distance  through  which  it  moves,  equals  the  weight 
multiplied  by  the  distance  through  which  it  moves. 

There  is  no  real  gain  of  mechanical  force  in  a  lever  or 
a  machine  of  any  kind.  A  machine  is  only  an  arrange- 
ment by  which  a  small  force  acting  through  a  great 
distance  is  converted  into  a  great  force  acting  through 
a  small  distance,  or  else  a  great  force  acting  through  a 
small  distance  is  converted  into  a  small  force  acting 
through  a  great  distance. 

When  a  small  force,  by  acting  through  a  great  dis- 
tance, is  made  to  raise  a  great  weight,  or  do  a  great 
deal  of  work,  there  is  said  to  be  a  gain  of  power  in  the 
machine.  When,  on  the  contrary,  a  great  force,  in  mov- 
ing through  a  small  distance,  lifts  only  a  small  weight, 
or  does  very  little  work,  there  is  said  to  be  a  loss  of 
power  in  the  machine.  But  whenever  there  is  a  gain 
in  power,  there  is  a  corresponding  loss  in  speed;  and 
whenever  there  is  a  loss  in  power,  there  is  a  corre- 
sponding gain  in  speed.  For  if,  in  the  machine,  a 
power  of  one  pound  is  made  to  move  a  weight  of  ten 
pounds,  then  the  weight  moves  only  one-tenth  as  fast  as 
the  power.  But  when  a  power  of  ten  pounds  is  made 
to  move  a  weight  of  one  pound,  then  the  weight  moves 
ten  times  as  fast  as  the  power. 

112.  Gain  and  Loss  of  Power  in  the  Lever.  —  In  a 
lever  of  the  first  kind,  when  the  fulcrum  is  just  half- 
way between  the  weight  and  power,  there  is  neither 
gain  nor  loss  in  power.  If  the  fulcrum  is  nearer  the 
weight  than  the  power,  there  is  a  gain  in  power  and 
a  loss  in  speed.  If  the  fulcrum  is  nearer  the  power 
than  the  weight,  there  is  loss  in  power  and  gain  in 
speed. 


MECHANICS.  77 

In  a  lever  of  the  second  kind,  the  power  is  always 
farther  from  the  fulcrum  than  the  weight,  and  con- 
sequently it  always  moves  through  greater  distance. 
Hence,  in  this  kind  of  lever,  there  is  always  a  gain  in 
power  and  a  loss  in  speed. 

In  a  lever  of  the  third  kind,  the  weight  is  always 
farther  from  the  fulcrum  than  the  power,  and  must 
move  the  greater  distance.  In  this  kind  of  lever,  then, 
there  is  always  a  loss  in  power  and  a  gain  in  speed. 

113.  The  Compound  Lever.  —  Sometimes  two  or  more 
simple   levers   are   combined,   as   shown   in  Figure  58. 

Fig  s8t  Suppose  that  P  be  five  times 

as  far  from  the  fulcrum  f  as 
A  is,  the  point  P  will  then 
move  five  times  as  fast  as  the 
point  A,  and  a  pull  of  one 
pound  on  P  will  exert  a  pull 
of  five  pounds  on  A.  If  £  is 
five  times  as  far  from  the  fulcrum  F  as  W  is,  the  five 
pounds  of  pull  on  J3  will  exert  twenty-five  pounds  of  pull 
at  W.  In  this  case,  one  pound  of  pull  exerted  at  P  will 
balance  twenty-five  pounds  at  W.  But  it  will  be  found 
on  trial  that  by  pulling  P  down  one  inch,  W  will  be 
raised  only  one  twenty-fifth  of  an  inch. 

Such  a  combination  of  levers  is  called  a  compound 
lever. 

114.  Bent  Levers.  —  Sometimes  the  arms  of  the  lever 
are  bent,  as  shown  in  Figure  59.     In  such  a  lever,  the 
lengths  of  the   arms   are  straight 

lines  drawn  from  the  fulcrum  at  Flg<  S9> 

right   angles   to   the   lines  which     ^^~~~^^£=--—'-yc 

show   the  direction  in  which  the  f" 

power  and  weight  act. 

The  common  claw-hammer,  as  used  for  drawing  nails, 
is  an  illustration  of  this  kind  of  lever. 


78 


MECHANICS. 


THE  WHEEL  AND  AXLE. 

115.   The  Rack  and  Pinion.  —  In  Figure  60,  we  have 
a  machine  called  the  rack  and  pinion.     The  crank  A 
turns  a  small  toothed  wheel  called  the 
pinion.     The  teeth  of  the  pinion,  one 
k     after  another,  catch  under  the  teeth  of 
\   an  upright  bar  B,  called  the  rack,  and 
each   tooth   raises  the  bar  a   little.      If 
the  rack  is  placed  under  the  weight,  it 
will  carry  up  the  weight  as  it  rises. 

1 1 6.  The  Rack  and  Pinion  is  a 
Modification  of  the  Lever.  —  In  the 
rack  and  pinion,  the  crank  takes  the  place  of  the  long 
arm  of  the  lever ;  the  rod,  or  axle,  upon  which  the 
pinion  turns  takes  the  place  of  the  fulcrum;  and  the 
pinion  takes  the  place  of  the  short  arm.  Each  tooth 
of  the  pinion  is,  in  fact,  the  short  arm  of  a  lever,  and  the 
pinion  is  a  contrivance  by  which  the  lever  is  furnished 
with  several  short  arms  instead  of  one.  These  short 
arms  act  upon  the  weight  one  after  another,  so  that  it 
can  be  raised  a  considerable  distance  without  interrup- 
tion. With  the  simple  lever,  it  is  evident  that  a  weight 
can  be  lifted  but  a  little  way  at  a  time. 

117.  The  Windlass.  —  In  the  windlass  (Figure  61), 
a  thick  axle,  or  barrel,  takes  the  place  of  the  pinion, 
and  a  rope  that  of  the  rack. 
When  the  crank  is  turned,  the 
rope  is  wound  upon  the  barrel, 
and  the  weight  raised.  In  one 
turn  of  the  crank,  the  rope  is 
wound  once  round  the  barrel, 
•*  and  the  weight  is  raised  a  dis- 
tance equal  to  the  circumference  of  the  barrel ;  while  the 


Fig.  61. 


MECHANICS.  79 

power  at  the  end  of  the  crank  moves  through  a  path 
like  the  dotted  line  in  Figure  60.  If  the  power  moves 
ten  times  the  distance  the  weight  moves  in  the  same 
time,  a  power  of  one  pound  at  the  end  of  the  crank 
ought  to  balance  ten  pounds  of  weight  hung  from  the 
barrel  (in). 

1 1 8.  The  Capstan.  —  In  the  windlass,  the  longer  the 
crank  and  the  smaller  the  barrel,  the  greater  the  gain  of 
power.  If,  however,  the  barrel  be  made  very  small,  it 
will  not  be  strong  enough ;  while  if  the  crank  be  made 
very  long,  it  cannot  be  conveniently  turned  with  the 
hand.  But  instead  of  one  crank  there  may  be  a  num- 
ber of  spokes ;  and,  if  the  barrel  be  placed  upright,  a 
man  may  pull  upon  one  spoke  after  another  as  they 
come  within  Ijis  reach,  and  thus  turn  the  barrel,  or 
several  men  may  'walk  round  it,  pushing  against  the 
spokes.  Such  an  upright  windlass,  with  long  spokes, 
is  called  a  capstan,  and  is  much  used  on  board  ships. 

Sometimes  the  capstan  (Figure  62)  is  arranged  with 
a  single  long  arm,  to  which  a  horse  can  be  harnessed. 

Fig.  62. 


119.    The  Wheel  and  Axle.  —  This  machine  is  simply 
a  wheel  with  a  thick  axle,  like  the  barrel  of  the  wind- 


8o 


MECHANICS. 


Fig.  63.  lass.     The  wheel  takes  the  place 

of  the  crank  of  the  windlass,  or 
the  spokes  of  the  capstan.  Power 
is  applied  to  the  wheel,  either  by 
means  of  pegs  upon  its  rim,  as  in 
Figure  63,  or  by  means  of  a  rope- 
or  band,  as  in  Figure  64. 

Suppose  that  the  circumference 
of  the  wheel  is  eight  times  that  of  the  axle.  If  we 
hang  a  weight  of  one  pound  to  the  wheel,  we  must 
hang  a  weight  of  eight  pounds  to  the  axle  in  order  to 
balance  it ;  and  the  former  will  move  eight  inches,  while 
the  latter  moves  one  (in). 

120.    The  Ratchet.  —  The  ratchet  is  an  arrangement 
to  keep  the  'wheel  from  turning  except  in  one  direc- 
tion.    It  consists  of  a  catch  c  (Figure  64),  which  plays 
Fig.  6*  into  the  teeth  of  the  wheel  A  B.     It 

thus  allows  the  wheel  to  turn  to  the 
left,  but  keeps  the  weight  from  pulling 
it  back  towards  the  right. 

121.  Wheel-work.  —  In  the  wheel 
and  axle,  the  larger  the  wheel  and  the 
smaller  the  axle,  the  greater  the  gain  of 
power.  But,  as  has  already  been  said 
(118),  if  the  barrel  be  made  very  small, 
it  may  not  be  strong  enough ;  and,  on  the  other  hand, 
if  the  wheel  be  made  very  large,  it  will  be  too  heavy  and 
take  up  too  much  room.  Instead  of  using  such  a  large 
wheel,  we  may  have  several  wheels  and  axles  acting 
upon  one  another,  like  the  levers  in  the  compound 
lever  (113).  Such  a  combination,  or  train,  of  wheels 
and  axles  is  often  called  wheel-work.  The  power  is 
applied  to  the  circumference  of  the  first  wheel,  and  the 
weight  is  hung  to  the  axle  of  the  last  wheel. 

Sometimes   one   wheel   turns    the    other   by   rubbing 


MECHANICS. 

Fig.  65. 


against  it,  or  by  friction.  The  most  common  way, 
however,  is  by  means  of  teeth  or  cogs  on  the  surfaces 
of  the  wheels  and  axles,  as  shown  in  Figure  65. 

If  the  teeth  project  from  the  side  of  Fig 

the  wheel,  as  in  Figure  66,  it  is 
called  a  crown-wheel.  If  their  edges 
are  sloped,  as  in  Figure  67,  the  wheel 
is  called  a  bevel-wheel.  Again,  the 
wheels  and  axles  may  be  made  to  act 
upon  one  another  by  means  of  a  belt, 
or  band,  passing  over  them  both. 

Fig.  67. 

J 


MECHANICS. 


They  may  thus  be  at  any  distance  apart,  and  may  turn 
either  the  same  way  or  contrary  ways,  according  as  the 
belt  does  or  does  not  cross  between  them. 


THE  PULLEY. 

122.  In  Figure  68,  we  have  a  fixed  ring  through  which 
passes  a  cord  with  a  weight  hung  to  it.      By  pulling 
down  the  cord  at  P,  the  weight  is  drawn  up.     It  is  often 
desirable  thus  to  change  the  direction  of  the  power. 
Fig.  68.  if  we  use  a  ring  for  this  purpose,  much  of 

the  power  will  be  wasted  by  the  friction,  or 
rubbing,  of  the  rope  against  the  ring.  We 
may  get  rid  of  much  of  this  friction  by  using, 
instead  of  the  ring,  a  pulley.  This  is  simply 
a  'wheel  'with  a  grooved  rim  to  keep  the  cord 
in  place. 
There  would  be  no  gain  in  power  by  the  use  of  the 

Fig.  69. 


MECHANICS.  83 

pulley.  It  is  evident  that  one  pound  on  one  side  of  the 
wheel  would  balance  just  one  pound  on  the  other  side ; 
and  that  if  the  former  were  drawn  down  one  inch,  the 
latter  would  be  drawn  up  just  one  inch. 

A  very  common  use  of  the  pulley  in  changing  the 
direction  of  the  power  is  illustrated  in  Figure  69. 

123.  Fixed  and  Movable  Pulleys.  —  In   Figure    70, 
the  frame  of  the  pulley  D  C  is  fastened  to  the  ceiling ; 
the  frame  of  the  pulley  AB  rises  as  the         Fig.  7o. 
rope  P  is  drawn  down.     A  pulley  like  D  C 

is  called  a  fixed  pulley;  one  like  A  B,  a 
movable  pulley.  The  frame  of  the  pulley 
is  often  called  the  block. 

124.  The  Law  of  the  Pulley.  —  In  the 
combination,  or  system,  of  pulleys  in  Fig- 
ure 70,   it  is   evident   that   the   rope    must 

have  the  same  tension,  or  strain  upon  it,  from  one  end 
to  the  other.  This  fact,  namely,  that  a  cord  •when 
stretched  must  have  the  same  strain  upon  it  through- 
out its  length,  is  called  the  law  of  the  pulley. 

125.  Systems  of  Pulleys  with  one  Rope.  —  In  Figure 
70,  the  tension  or  strain  of  the  rope  is  equal  to  the  power 
P,  since  it  balances  the  power.      If  a  weight   of  one 
pound  is  hung  to  the  rope  at  P,  there  will  be  a  strain  of 
one  pound  on  the  part  of  the  rope  on  that  side  of  the 
pulley.     There  must  then  be  a  strain  of  one  pound  upon 
the  part  of  the  rope  between  A  and  D,  and  a  strain  of 
one  pound  between   3  and   H.     These   two   tensions, 
AD  and  B H,  will  evidently  sustain  a  weight  of  two 
pounds  at  W.     In  this  system  of  pulleys,  then,  a  power 
of  one  pound  balances  a  weight  of  two  pounds. 

But  here,  as  in  every  other  machine  (m)5  what  is 
gained  in  power  is  lost  in  speed.  If  the  power  P 
is  drawn  down  one  foot,  the  weight  W  will  rise  only 
half  a  foot ;  for  of  the  one  foot  added  to  the  length  of 


84 


MECHANICS. 


one-half  will  be  taken  from  AD  and  one-half 
from  B  H. 

In  the  system  of  pulleys  shown  in  Figure  71,  we  see 
that  one  pound  at  P  will  balance  three  pounds  at  W, 

Fig.  71. 


since  each  of  the  three  parts  of  the  rope  on  that  side 
of  the  pulley  C  has  a  tension  of  one  pound.  But  P 
must  be  drawn  down  three  feet  in  order  to  raise  W 
one  foot. 

In  Figure  72,  we  have  a  system  of  pulleys  in  which 
the  weight  is  four  times  the  power;  and  in  this  case 
the  power  evidently  moves  four  times  as  far  as  the 
weight. 

126.  Systems  of  Pulleys  'with  more  than  one  Rope. 
—  Figure  73  represents  a  system  of  pulleys,  in  which 
two  ropes  are  used.  Here  a  weight  of  four  pounds  is 
balanced  by  a  power  of  one  pound.  The  parts  of  the 
rope  A  D  and  A  B  must  each  have  a  tension  equal  to  the 
power.  The  rope  A  C  B  balances  the  two  tensions,  B  P 
and  B  A,  and  must  therefore  have  a  tension  of  twice  the 
power.  The  three  tensions  supporting  the  pulley  A 
amount  therefore  to  four  times  the  power. 


MECHANICS. 


In  the  system  shown  in  Figure  74,  four  ropes  are  used. 
The  tensions  of  the  several  ropes  will  be  readily  under- 

Fig.  73.  Fig.  74. 


stood  from  the  numbers.      In  this  case,  the  power  is 
doubled  by  each  movable  pulley  which  is  added. 


THE  INCLINED  PLANE. 

127.  When  a  heavy  cask  is  to  be  raised  into  a  cart  or 
dray,  a  ladder  is  often  used.     One  end  of  the  ladder  is 
placed  upon  the  cart  behind  and  the  other  end  upon  the 
ground,  and  the  cask  is  rolled  up  the  inclined  surface  thus 
formed.      One  man  may  thus   raise   a  load  of   several 
hundred  weight  with  comparative  ease.      An   inclined 
surface  used  in  this  way  is  called  an  inclined  plane. 

We  often  see  inclined  planes  on  a  large  scale  in  roads. 

128.  The  Law  of  the  In-  Fig  7S 
dined  Plane   the  same   as 

that  of  other  Machines.  — 
In  Figure  75,  we  have  an 
inclined  plane.  W  is  the 
weight,  which  is  balanced 
by  the  power  P.  B  C  is  the  height  of  the  inclined 


86  MECHANICS. 

plane,  and  A  C  is  its  length.  It  is  evident  that  the 
power  must  descend  a  distance  equal  to  the  length  of  the 
inclined  plane,  in  order  to  raise  the  weight  a  distance 
equal  to  its  height.  Now  it  is  found  on  trial  that,  if  the 
length  of  the  inclined  plane  is  sixteen  feet,  and  its  height 
four  feet,  a  power  of  one  pound  will  balance  four  pounds 
of  weight.  But  one  multiplied  by  sixteen  equals  four 
multiplied  by  four;  that  is,  the  power  multiplied  by 
the  distance  through  'which  it  acts  equals  the  weight 
multiplied  by  the  distance  through  which  it  is  raised. 
It  follows  from  the  above,  that  the  greater  the  length  of 
the  inclined  plane,  compared  with  its  height,  the  less 
the  force  necessary  to  raise  a  weight,  and  the  slower 
the  weight  rises. 

THE    WEDGE. 

129.  Instead  of  lifting  a  weight  by  moving  it  along 
an  inclined  plane,  we  may  do  the  same  thing  by  push- 
ing the  inclined  plane  under  the  weight.  When 
thus  used,  the  movable  inclined  plane  is  called  the 
wedge.  A  wedge  which  is  used  for  splitting  wood  has 
p.  6  usually  the  form  of  a  double  inclined  plane, 
as  in  Figure  76.  The  law  of  the  wedge  is 
the  same  as  that  of  the  inclined  plane; 
but  since  a  wedge  is  usually  driven  by  a  blow 
instead  of  a  force  acting  continuously,  it  is 
difficult  to  illustrate  this  law  by  experiments. 
130.  Uses  of  the  Wedge.  —^  The  wedge  is 
especially  useful  when  a  large  weight  is  to 
be  raised  through  a  very  short  distance. 
Thus  a  tall  chimney,  the  foundation  of 
which  has  settled  on  one  side,  has  been  made  upright 
again  by  driving  wedges  under  that  side.  So,  too, 
ships  are  often  raised  in  docks  by  driving  wedges  under 


MECHANICS. 


Fig.  77. 


their  keels.  Cutting  and  piercing  instruments,  such 
as  razors,  knives,  chisels,  awls,  pins,  needles,  a.nd  the 
like,  are  different  forms  of  wedges. 

THE    SCREW. 

131.  The  screw  (Figure  77)   is  a  movable  inclined 
plane,    in  which   the  inclined  surface    winds    round 
a  cylinder.     The   cylinder  is  the 

body  of   the    screw,    and   the   in- 
clined surface  is  its  thread. 

The  screw  usually  turns  in  a 
block  N,  called  the  nut.  Within 
the  nut  there  are  threads  exactly 
corresponding  to  those  on  the 
screw.  The  threads  of  the  screw 
move  between  those  of  the  nut. 

The  power  is  usually  applied  by 
means  of  a  lever  P.     Sometimes 
the  screw  is  fixed  and  the  nut  is 
movable,  and  sometimes  the  nut  is  fixed  and  the  screw 
movable. 

132.  The  Endless  Screw.  —  In  Fig- 
ure 78,  the  thread  of  the  screw  works 
between   the   teeth   of  the   wheel,    and 
turns  it.     Since  as  fast  as  the  teeth  at 
the  left  escape  from  the  screw  those  on 
the  right  come  up  to  it,  the  screw  acts 

on  the  wheel  continually  ;  hence  the  name  of  the  machine. 


Fig.  78. 


SUMMARY. 

A  machine  is  a  contrivance  by  which  force  is  made  to 
do  work.  In  a  machine  there  is  no  real  gain  of  force, 
but  a  force  may  be  changed  in  direction,  and  a  small  force 


88  MECHANICS. 

acting  through  a  great  distance  may  be  converted  into  a 
large  force  acting  through  a  small  distance,  or  a  large 
force  acting  through  a  small  distance  converted  into  a 
small  force  acting  through  a  great  distance.  (109,  in.) 

The  first  simple  machine  is  the  lever.  There  are  three 
kinds  of  levers,  depending  upon  the  relative  position  of 
the  weight,  the  fulcrum,  and  the  power.  (109.) 

In  every  machine,  the  power  and  weight  will  balance 
each  other  when  the  power  multiplied  by  the  distance 
which  it  moves  is  equal  to  the  weight  multiplied  by  the 
distance  which  it  moves  in  the  same  time,  (m.) 

A  compound  lever  is  a  machine  in  which  two  or  more 
simple  levers  are  combined.  (113.) 

The  rack  and  pinion  is  a  lever  whose  short  arm  is 
multiplied  in  the  pinion.  (116.) 

In  the  windlass,  the  barrel  and  the  rope  take  the  place 
of  the  pinion  and  the  rack.  (117.) 

In  the  wheel  and  axle,  the  long  arm  of  the  lever  is  mul- 
tiplied as  well  as  the  short  one.  (119.) 

Several  wheels  are  often  combined  so  as  to  act  upon 
one  another.  The  wheels  may  be  made  to  act  upon  one 
another  by  means  of  cogs,  or  by  means  of  belts.}  (121.) 

The  direction  in  which  a  force  acts  may  be  changed  by 
means  of  a  single  fixed  pulley.  (122.) 

In  a  system  of  pulleys,  the  mechanical  advantage  de- 
pends upon  the  fact  that  a  stretched  rope  will  have  the 
same  tension  throughout  its  whole  length.  (124.) 

The   fourth   simple   machine   is   the   inclined  plane. 


The  fifth  simple  machine  is  the  wedge.  This  is  really 
a  movable  inclined  plane  which  is  pushed  under  the 
weight  to  be  raised.  (129.) 

The  sixth  simple  machine  is  the  screw.  This  is  also 
a  movable  inclined  plane  arranged  round  a  cylinder. 

(131.) 


MECHANICS. 


WATER  POWER. 


89 


Fig.  79- 


J33'  Water  Wheels. — An  important  source  of  me- 
chanical power  is  falling  water.  The  falling  or  run- 
ning water  is  made  to  turn  a  wheel,  called  a  'water 
wheel,  and  this  wheel  is  made  to  drive  machinery. 

Water  wheels  are  of  various  forms.  Some  turn  on 
an  upright  axis,  and  others  on  a  horizontal  axis. 
The  latter  are  called  vertical  water  wheels  ;  and  the 
former,  horizontal  water  wheels. 

134.  Vertical  Water  Wheels.  —  One  of  the  most 
common  forms  of  vertical  water  wheels  is  the  breast- 
wheel,  represented  in  Figure  79.  It  consists  of  a  series 
of  boxes,  or  buckets,  arranged  on  the  outside  of  a  wheel 
or  cylinder.  Water  is  al- 
lowed to  flow  into  these 
buckets  on  one  side  of 
the  wheel,  and  by  its 
weight  causes  the  wheel 
to  turn.  The  buckets  are 
so  constructed  that  they 
hold  the  water  as  long 
as  possible  while  they 
are  going  down,  but  al- 
low it  all  to  run  out  be- 
fore they  begin  to  rise 
on  the  other  side. 

The  overshot  wheel  is  similar  to  the  breast-wheel  in 
all  respects,  except  that  the  water  is  led  over  the  top  of 
the  wheel  and  poured  into  the  buckets  on  the  other  side. 

The  undershot  wheel  has  boards  projecting  from  its 
circumference,  like  the  paddle-wheel  of  a  steamboat. 
The  water  runs  under  the  wheel,  and  turns  it  by  the 
force  of  the  current  pressing  against  the  boards. 


9O 


MECHANICS. 


Fi 


135.  Barkers  Mill.  —  In  Figure  80,  we  have  a  hol- 
low upright  cylinder,  with  two  horizontal  arms  at  the 
bottom,  and  turning  on  an  axis.  The 
cylinder  is  open  at  the  top,  but  closed 
below,  except  that  it  has  two  holes  on 
opposite  sides  of  the  arms  near  the 
end,  as  shown  in  the  figure.  If  water 
be  poured  in  at  the  top,  the  cylinder 
begins  to  turn  round,  and  will  continue 
to  turn  as  long  as  the  supply  of  water 
1S  k6?*  UP*  ^  tne  holes  in  the  arms 
are  stopped  up,  the  cylinder  ceases  to 
move.  This  apparatus  is  known  as 
Barker's  mill.  Its  action  is  easily  understood  when  we 
recollect  that  liquids  press  equally  in  all  directions 
(56).  If  the  holes  in  the  arms  are  plugged  up,  the 
water  presses  forward  against  the  plug  ;  and  it  presses 
backward  against  the  opposite  part  of  the  arm  with  an 
equal  force,  so  that  there  will  be  no  motion.  If  now 
we  remove  the  plug,  there  will  be  no  pressure  against 
that  part  of  the  arm  to  balance  the  backward  pressure 
against  the  opposite  side  ;  and  the  arm  consequently 
turns  backward.  As  the  holes  in  the  arms  are  gn  oppo- 
site sides  of  the  tube,  the  backward  pressure  on  each 
arm  tends  to  turn  the  cylinder  in  the  same  direction. 
Fis-  8l-  This  machine  gains  in  power  by 

curving  the  arms  (Figure  81)  ;  for  the 
water  is  thus  made  to  press  more 
powerfully  against  the  bend  of  the  arm 
as  it  flows  through  the  tube. 
136.  The  Turbine  Wheel.  —  The  power  of  Barker's 
mill  would  evidently  be  increased  by  increasing  the  num- 
ber of  the  arms.  Instead  of  these  arms  we  might  have 
curved  partitions  placed  between  two  flat  disks,  forming 
a  wheel  (Figure  82). 


MECHANICS.  91 

Suppose  now  that  the  wheel 
were  cut  round  where  the  dotted 
circle  is  seen  in  the  figure, 
and  that  the  outer  part  were 
arranged  to  turn  round  freely 
while  the  central  part  was  kept 
stationary.  If  water  were  poured 
into  the  wheel  from  above,  the 
outer  part  would  of  course  turn 
round  just  as  the  whole  wheel 
did  before  it  was  cut.  For  the  action  of  the  water 
against  the  partitions  would  evidently  be  the  same  as 
before,  and  it  was  this  action  which  turned  the  wheel. 

The  central  partitions  may  be  so  curved  that  the  water 
will  strike  the  outer  partitions  nearly  at  right  angles. 
The  direct  force  of  the  current  is  thus  very  much  in- 
creased, and  may  become  far  greater  than  the  reactionary 
force  already  explained.  This  is  the  arrangement  in  the 
turbine  wheel^  the  most  efficient  water  wheel  yet  devised. 


STEAM  POWER. 


137.  The  Steam  Engine.  —  The  elastic  force  of  con- 
fined steam  (71,  82)  can  be  made  to  work  a  piston. 

The  steam  coming  from  the  boiler  by  the  tube  x 
(Figure  83)  passes  into  the  box  d.  From  this  box  two 
pipes,  a  and  £,  carry  the  steam,  one  above  and  the  other 
below,  the  piston.  A  sliding  valve  y  is  so  arranged  that 
it  always  closes  one  of  these  pipes.  In  the  right-hand 
figure,  the  lower  pipe  b  is  open,  and  the  steam  can  pass 
in  under  the  piston  and  force  it  up.  At  the  same  time, 
the  steam  which  has  done  its  work  on  the  other  side  of 
the  piston  passes  out  through  the  pipes  a  and  O. 

The  sliding  valve  is  connected  by  mea'ns  of  the  rod  i 
with  the  crank  of  the  engine,  so. that  it  moves  up  and 


92 


MECHANICS. 


down  as  the  piston  moves  down  and  up.  As  soon,  then, 
as  the  piston  has  reached  the  top  of  the  cylinder,  the 
sliding  valve  is  brought  into  the  position  shown  in  the 

Fig.  83. 


left-hand  figure.  The  steam  now  passes  into  the  cylinder 
above  the  piston  through  the  pipe  a  and  forces  the  piston 
down,  and  the  steam  on  the  other  side  which  has  done 
its  work  goes  out  through  b  and  O.  The  sliding  valve 
is  now  again  in  the  position  shown  in  the  right-hand 
figure,  and  the  piston  is  driven  up  again  as  before ;  and 
thus  it  keeps  on  moving  up  and  down,  or  in  and  out. 
This  kind  of  motion  is  called  reciprocating  motion. 

Fig.  84.  In   using   the  engine   for  doing 

work,  it  is  generally  necessary  to 
change  this  reciprocating  motion 
into  a  rotary  one ;  that  is,  to 


MECHANICS. 


93 


make  the  piston,  as  it  moves  up  and  down,  turn  a 
wheel.  This  is  usually  done  by  means  of  a  crank. 
The  crank  is  sometimes  connected 
with  the  piston-rod  directly,  the  cyl- 
inder being  placed  either  horizontally, 
as  shown  in  Figure  84,  or  upright,  as 
in  the  engine  represented  in  Figure 
86.  In  other  cases,  the  piston-rod  turns 
the  crank  by  means  of  a  ivalking- 
beam,  the  arrangement  and  action  of  ~J 
which  will  be  evident  from  Figure  85.  The  walking- 
beam  is  much  used  for  large  engines,  especially  on 
steamboats. 

In  Figure  86,  we  have  a  picture  of  a  small  stationary 
steam-engine,  which  shows  how  the  parts  of  the  ma- 
chine already  described  are  put  together,  and  also  illus- 
trates those  parts  which  have  not  yet  been  mentioned. 

On  the  right  is  the  cylinder  P,  which  is  supplied  with 
steam  from  the  boiler  by  the  pipe  x.  TJie  waste  steam  is 
carried  away  by  the  pipe  L.  Within  the  cylinder  is  the 
piston  moving  up  and  down  as  explained  above.  The 
piston-rod  A  moves  the  crank  M,  and  thus  turns  the  axle 
D,  which  may  be  connected  with  the  machinery  to  be 
driven,  by  means  of  a  belt  X,  as  here,  or  by  a  train  of 
wheels,  or  in  various  other  ways.  J^>  is  a  pump,  like  that 
shown  in  Figure  45,  which  supplies  the  boiler  with  water, 
through  the  pipe  R.  It  is  worked  by  the  engine  itself 
by  means  of  the  rod  g  and  the  cam,  or  eccentric,  E. 

138.  The  Governor.  —  The  governor  is  a  contrivance 
by  which  the  engine  regulates  its  own  speed,  so  that  it 
may  not  be  too  suddenly  quickened  or  retarded  by  vari- 
ations in  the  work  to  be  done.  It  consists  of  two  arms,  k  r 
(Figure  86),  carrying  heavy  iron  balls,  m,  n,  at  one  end, 
and  attached  by  joints  at  the  other  end  to  the  rod  c.  The 
whole  is  made  to  rotate  by  means  of  the  bevel-wheels  a 


94 


MECHANICS. 


and  b  (121),  which  are  turned  by  the  engine  itself.     If 
the  speed  of  the  engine  is  quickened,  the  governor  rotates 

Fig.  86. 


MECHANICS.  95 

faster,  and  the  arms  and  balls  tend  to  separate  more  and 
more ;  just  as  two  balls  hung  side  by  side  will  do  when 
the  strings  by  which  they  are  held  are  twirled  by  the 
hand.  As  the  arms  spread  out,  they  raise  the  ring  r, 
which  slides  freely  on  the  rod  c;  and  as  r  rises,  it  acts 
upon  the  levers  s,  /,  and  O,  which  partially  close  a  valve 
in  the  pipe  x.  This  valve  is  seen  at  v  in  Figure  83.  The 
supply  of  steam  from  the  boiler  is  thus  diminished,  and 
the  speed  of  the  engine  is  retarded.  The  governor  now 
rotates  less  rapidly,  the  arms  drop  a  little,  the  ring  r 
slides  down,  the  valve  in  x  is  opened  a  little  more,  let- 
ting steam  pass  to  the  cylinder  more  freely,  and  the 
speed  of  the  engine  is  quickened  again.  Thus  any  ten- 
dency to  go  faster  or  slower  corrects  itself  very  promptly ; 
and  the  engine  runs  at  almost  exactly  the  same  speed, 
however  much  the  resistance  may  vary. 

139.  The  Fly -Wheel. — As  the  crank  turns  round,  it 
will  be  seen  that  there  are  two  points  where  the  pis- 
ton-rod is  pushing  exactly  in  the  direction  of  the  point 
round  which  the  crank  moves ;  and  that  at  these  points 
it  does  not  tend  to  turn  the  crank  at  all.     There  must 
therefore  be  some  means  of  carrying  the  crank  past  these 
dead  points,  as  they  are  called.     This  is  the  office  of  the 

fly-ivheel  V,  a  heavy  iron  wheel  attached  to  the  axle  D. 
The  great  momentum  (99)  of  this  heavy  mass  tends  to 
carry  the  axle  round  with  a  uniform  motion,  notwith- 
standing the  variations  in  the  power  acting  upon  it. 

140.  High  Pressure  and  Loiv  Pressure  Engines.  — 
When  the  steam,  after  doing  its  work  in  the  cylinder,  is 
carried  into  a  cold  chamber,  the  engine  is  said  to  be  of 
low  pressure;  when  it  is  forced  out  into  the  air,  the 
engine  is   said  to  be  of  high  pressure.     In  the  former 
case,   the  steam   is  condensed  into  'water  in  the  cold 
chamber,   and  a   vacuum    is  thus  formed  behind  the 
piston.     In  the  latter  case,  the  piston  has  to  act  against 


96  MECHANICS. 

the  pressure  of  the  atmosphere,  which,  as  we  have 
learned  (74),  is  equivalent  to  a  weight  of  15  pounds 
on  each  square  inch  of  its  surface.  It  is  evident  that  a 
greater  pressure  of  steam  will  be  necessary  to  move  the 
piston  in  the  latter  case. 

141.  The  Boiler.  —  In  the  boiler,  the  steam  is  pro~ 
duced,  and  confined  until  it  is  used  in  moving  the 
piston. 

Boilers  are  usually  made  of  plates  of  wrought  iron  or 
copper  riveted  together.  Copper  is  the  best  material,  but 
iron  is  generally  used  on  account  of  its  cheapness. 

In  order  to  get  the  full  effect  of  the  fire,  the  hot  gas 
and  smoke  from  it  are  usually  made  to  pass  through 
Jlues  or  tubes  in  the  body  of  the  boiler;  and  the  water 
comes  directly  in  contact  with  these  flues  or  tubes.  This 
is  illustrated  in  the  Cornish  boiler,  as  it  is  called,  shown 
in  Figure  87,  and  considered  one  of  the  best  forms  of 
Fig.  87.  boiler.  It  is  a  cylinder, 

frequently  more  than  forty 
feet  long,  and  from  five  to 
seven  feet  in  diameter,  with 
two  cylindrical  flues,  B  B, 
extending  its  whole  length. 
These  flues  serve  as  the  fur- 
nace in  which  the  fire  is  built.  The  hot  gas  and  smoke, 
after  passing  through  the  flues,  circulate  round  the  out- 
side of  the  boiler  before  escaping  into  the  chimney. 

Figure  88  represents  the  usual  form  of  the  boiler  of  a 
locomotive  engine.  The  furnace,  or  Jire-box,  A,  is  with- 
in the  boiler,  and  is  surrounded  by  water  except  beneath 
and  at  the  door  D.  A  large  number  of  stout  tubes  extend 
from  the  fire-box  through  the  boiler  to  the  smoke-box  1$. 
The  hot  gases  and  smoke  pass  through  these  before  they 
escape  into  the  chimney.  E  is  the  steam-dome,  from  the 
top  of  which  a  large  tube  conveys  the  steam  into  the 


MECHANICS. 


97 


chamber  F,  from  which  it  passes  by  tubes  on  each  side 
to  the  cylinders.     The  waste  steam  from  the  cylinders 

Fig.  88. 


passes  into  the  chimney  through  two  pipes  meeting  at  K, 
and  thus  increases  the  draught  of  the  furnace. 

142.  The  Locomotive  Engine.  —  This  machine  is 
shown  in  full  in  Figure  89.  The  boiler  XX  has  just 
been  described.  D  is  the  fire-box ;  Y,  the  smoke-box ; 
# ,  the  tubes  connecting  the  two  ;  0,  the  door  for  putting 
in  fuel ;  H,  the  vent-cock,  by  which  the  water  can  be 
drawn  off  from  the  boiler  ;  R  R,  the  feeders  which  con- 
duct water  from  the  tender  to  two  force-pumps  (not  seen 
in  the  figure)  by  which  it  is  forced  into  the  boiler.  At 
i  are  the  safety-valves,  kept  down  by  spiral  springs  in 
the  cases  e.  When  the  pressure  of  steam  in  the  boiler 
becomes  too  great  for  safety,  the  valves  open  and,  by 
allowing  a  part  of  the  steam  to  escape,  reduce  the  pres- 
sure, g  is  the  steam-whistle;  G,  a  rod  which  controls 
the  valve  /,  by  which  steam  is  let  into  the  steam-pipe  A. 
The  engineer  is  represented  as  holding  in  his  hand  the 
lever  by  which  this  valve  is  opened  more  or  less,  to 
regulate  the  speed  of  the  engine.  The  steam-tube  A 
passes  through  the  boiler,  as  shown  by  the  dotted  lines, 
into  the  smoke-box,  where  it  branches  off  to  the  two 
cylinders.  In  this  engine  there  is  no  chamber,  like  that 

7 


MECHANICS. 


marked  F,  in  Figure  '88.     One  of  the  cylinders  is  seen 
at  F,  laid  open  to  show  the  piston  P.     The  sliding  valve 

rig.  89. 


MECHANICS. 


99 


by  which-  the  steam  is  admitted  to  the  cylinder  is  pre- 
cisely like  the  one  figured  and  described  above  (137)  ; 
but,  being  behind  F  under  the  boiler,  it  does  not  appear 
here.  E  is  the  pipe  by  which  the  waste  steam  is  dis- 
charged into  the  smoke-pipe  J^.  K  is  the  connecting- 
rod,  by  means  of  which  the  piston  turns  the  crank  M  on 
the  axle  of  the  driving-wheels.  In  starting  the  engine, 
the  valves  must  be  moved  by  hand.  This  is  done  by 
means  of  the  lever  J3  and  the  rod  C.  1 1  are  stop-cocks, 
through  which  any  water  condensed  in  the  cylinders  can 
be  driven  out ;  v,  the  rod  for  opening  these  cocks. 
The  locomotive  is  always  a  high  pressure  engine. 


SUMMARY. 

The  downward  and  lateral  pressure  of  water  is  a 
source  of  mechanical  power.  (133.) 

The  downward  pressure  of  water  is  made  to  turn  a 
vertical  water  wheel.  ( 1 34. ) 

The  lateral  pressure  of  water  is  made  to  turn  a  hori- 
zontal or  reaction  water  wheel.  The  turbine  wheel  is 
a  reaction  wheel,  and  the  most  efficient  water  wheel 
known.  (135,  136.) 

The  elastic  force  of  steam  is  used  as  a  source  of 
mechanical  power  in  the  steam  engine.  (137-) 

The  essential  parts  of  the  steam  engine  are  the  boiler, 
in  which  the  steam  is  generated  ;  the  cylinder,  in  which 
the  expansive  force  of  the  steam  is  made  to  work  a 
piston ;  and  the  crank,  by  which  the  motion  of  the 
piston  is  made  to  turn  a  shaft,  (141?  I37-) 

In  the  low  pressure  engine,  a  vacuum  is  formed  be- 
hind the  piston  by  condensing  the  steam  which  has 
been  used ;  in  the  high  pressure  engine,  this  steam  is 
forced  out  against  the  pressure  of  the  air.  (140.) 


SOUND. 


NATURE  AND  PROPAGATION  OF  SOUND. 

143.  A  Sounding  Body  is  a  Vibrating  Body.  —  If  a 
glass  bell-jar  held  by  the  knob  be  struck  with  the 
knuckle,  it  gives  out  a  sound.  If  a  bit  of  metal,  ivory, 
or  other  hard  substance  be  placed  within  the  bell,  as 
seen  in  Figure  90,  it  is  tossed  up  and  down  rapidly, 
showing  that  the  bell  is  vibrating. 

Fig.  90. 


Every  body  vibrates  while  giving  out  sound^  and 
it  is  only  by  causing  a  body  to  vibrate  that  it  can  be 
made  to  give  out  sound. 

144.  Sound  will  not  pass  through  a  Vacuum.  —  In 
Figure  91,  the  bell  B  is  suspended  by  silk  threads  under 
the  receiver  of  the  air-pump.  The  bell  is  struck  by 
means  of  clock-work,  set  in  motion  by  the  sliding  rod  r. 
If  the  bell  be  struck  before  exhausting  the  air,  it  can  be 
distinctly  heard ;  but  as  the  air  is  exhausted,  the  sound 
becomes  fainter  and  fainter,  until  at  last  it  can  hardly 
be  perceived  even  with  the  ear  close  to  the  receiver. 
Sound,  then,  cannot  pass  through  a  vacuum. 


SOUND. 


101 


The  slight  sound  which  is 
heard  is  transmitted  by  the 
little  air  left  in  the  receiver, 
and  by  the  cords  which  hold 
up  the  bell. 

145.  Sound  passes  through 
all  Gases.  —  If  hydrogen  or 
any   other   gas    be    now   al- 
lowed  to   pass   into    the    re- 
ceiver, the  sound  of  the  bell 
will  be  heard  again. 

146.  Sound  passes  through 
Liquids  and  Solids.  —  If  a 
bell  be  put  under  water  and 
struck,  it  can  be  heard.     If 
a  person  puts  his  ear   close 
to  the  rail  of  an  iron  fence, 
and    the    rail    be    struck    at 
a    considerable    distance,   he 
hears  the  blow  twice.     The 
first    sound    comes    through 

the  rail ;  the  second,  which  soon  follows,  comes  through 
the  air.  These  experiments  show  that  sound  passes 
through  liquids  and  solids. 

A  slight  scratch  upon  the  iron  rail,  which  could  not 
be  heard  at  all  through  the  air,  is  heard  distinctly  when 
the  ear  is  placed  against  the  rail ;  showing  that  the  solid 
transmits  the  sound  better  than  the  air.  If  the  ear  be 
placed  near  the  ground,  the  tramp  of  horses  or  the  tread 
of  men  can  be  heard  at  a  great  distance,  the  sound  being 
conveyed  by  the  solid  earth. 

A  vibrating  body  throws  the  molecules  of  air,  or 
other  elastic  medium  around  it,  into  vibration,  and 
these  vibrations  are  sent  on  from  molecule  to  mole- 
cule until  they  reach  the  ear. 


IO2  SOUND. 

147.  The   Intensity   of    Sound    depends    upon    the 
Amplitude    of   the     Vibrations.  —  If    the    bell-jar    in 
Figure  90   be    struck    lightly,   it  will   give   out   a   faint 
sound,  and  the  bit  of  metal  will  be  but  slightly  agitated ; 
if  it  be  struck  a  harder  blow,  it  will  give  out  a  louder 
sound,  and  the  metal  will  be   more  violently  agitated. 
It  is  evident  that  in  the  latter  case  the  bell-jar  moves 
backward  and  forward  through  a  greater  space  than 
in  the  former ;    in  other  words,   that  the  amplitude  of 
its  vibrations  is  greater.     The  intensity,  or  loudness,  of 
sound,  then,  depends  upon  the  amplitude  of  the  vibra- 
tions  of  the  sounding  body. 

148.  The    Intensity   of  Sound    diminishes    as    the 
Square  of  the  Distance  of  the   Sounding  Body  in- 
creases.—  If  we   place   a   bell  ten  yards  off,   and  four 
bells  of  the  same  size  twenty  yards  off,  we  shall  find 
that  the  sound  of  the  one  bell  will  be  just  equal  to  that 
of  the  four  bells.     At  the  distance  of  thirty  yards,  nine 
bells  would  be  necessary  to  produce  a  sound  equal  to 
that  of  the  one  bell  at  ten  yards.     Sound,  then,  dimin- 
ishes in  intensity  as  the  square  of  the  distance  from 
the  sounding  body  increases. 

149.  Speaking -Tubes.  —  If  the  sound   is   prevented 
from    spreading   in   all   directions,  it   loses   little    of  its 
intensity.      Thus  Biot  found  that,  through  one   of  the 
water-pipes  of  Paris,  words  spoken  in  a  very  low  tone 
could  be  heard  three-quarters  of  a  mile  off.     The  sides 
of  the  pipe  kept  the  sound  from  spreading.     Conversa- 
tion can  be  carried  on  between  distant  parts  of  a  large 
building  by   means   of    small    tubes,    called    speaking- 
tubes. 

150.  Sound  travels  through  the  Air  at  the  Rate  of 
i  ,090  Feet  a  Second.  —  The  velocity  of  sound  in  air  has 
been  several  times  determined  by  experiment.     In  1822, 
the  French  Board  of  Longitude  chose  two  heights  near 


SOUND. 


I03 


Paris,  and  from  the  top  of  each  fired  a  cannon  at  inter- 
vals of  ten  minutes  during  the  night.  The  time  between 
seeing  the  flash  and  hearing  the  report  was  carefully 
noted  at  both  stations,  and  the  average  of  the  results 
showed  that  sound  travels  through  the  air  at  the  rate  of 
1,090  feet  a  second.  In  such  experiments,  the  time 
taken  by  the  light  to  pass  between  the  stations  is  too 
small  to  be  perceived. 

151.  The  Velocity  of  Sound  in  Water  is  about  4,700 
Feet  a  Second.  —  This  was  determined  at  the  Lake  of 
Geneva,  in  1826,  by  Colladon  and  Sturm.     They  found 
that,  when  a  bell  was  struck  under  water  on  one  side  of 
the  lake,  the  sound  could  be  distinctly  heard  at  a  distance 
of  nine  miles  on  the  other  side  by  putting  the  ear  to  one 
end  of  a  tube  whose  other  end  was  in  the  water.     It  was 
thus  found  that  the  velocity  of  sound  in  water  is  about 
^floofeet  a  second. 

152.  Sound    travels    through    Solids  faster    than 
through  Air.  —  It  is  found  by  the  experiment  with  the 
iron  rail    mentioned   above   (146)   that   the   velocity  of 
sound  in  a  solid  body  is  greater  than  in  the  air. 

153.  Sound  is  rejlected  on  meeting  a  new  Medium. 
—  Experiments  show  that  when  sound  meets  a  new 
medium,  it  is  reflected ;  and  that,  as  in  reflected  motion 
(102),  the  angle  of  reflection  is  equal  to  the  angle  of 
incidence.  -^y^ 

154.  Echoed—  When  there  is  a  sufficient  interval 
between  the  direct  and  the  reflected  sound,  we  hear  the 
latter  as  an  echo.  The  reflected  sound  has  the  same 
velocity  as  the  direct  sound,  so  that  the  echo  of  a  pistol- 
shot  from  the  face  of  a  cliff  1,090  feet  distant  is  heard 
two  seconds  after  the  explosion. 

An  echo  in  Woodstock  Park  repeats  seventeen  syllables 
by  day,  and  twenty  by  night ;  one  on  the  banks  of  the 
Lago  del  Lupo,  above  the  fall  of  Terni,  repeats  fifteen. 


104  SOUND. 

In  the  whispering  gallery  of  St.  Paul's,  the  faintest  sound 
is  conveyed  from  one  side  of  the  dome  to  the  other,  but  is 
not  heard  at  any  point  between.  At  Carisbrook  Castle, 
in  the  Isle  of  Wight,  is  a  well,  210  feet  deep  and  12  wide, 
lined  with  smooth  masonry.  When  a  pin  is  dropped  into 
the  well,  it  is  distinctly  heard  to  strike  the  water. 

In  some  cases,  the  sound  is  reflected  several  times,  and 
a  succession  of  echoes  is  heard,  each  feebler  than  the  pre- 
ceding, since  a  part  of  the  sound  is  lost  at  each  reflection. 

Sounds  are  also  reflected  from  the  clouds.  When  the 
sky  is  clear,  the  report  of  a  cannon  on  an  open  plain  is 
short  and  sharp  ;  while  a  cloud  is  sufficient  to  produce  an 
echo  like  the  rolling  of  distant  thunder.  A  feeble  echo 
also  occurs  when  sound  passes  from  one  mass  of  air  to 
another  of  different  density. 


SUMMARY. 

Sound  originates  in  a  vibrating  body.     (143.) 

It  is  not  propagated  through  a  vacuum.     (144.) 

It  is  propagated  through  all  elastic  substances,  whether 

gases,  liquids,  or  solids,  by  vibrations  of  their  molecules. 

(145,  146.) 
Its  intensity  increases  with  the  amplitude  of  the  vibra- 

tions, and  diminishes  as  the  square  of  the  distance  from 

the  sounding  body  increases.     (147,  148.) 

The  velocity  of  sound  in  air  is   1,090  feet  a  second. 

(150.) 

The  velocity  of  sound  in  water  is  about  4,700  feet  a 
second.     Its  velocity  in  solids  is  greater  than  in  the  air. 


On   meeting  a  different  medium,  sound  is  reflected. 

(153-) 

Echoes  are  due  to  reflected  sound.     (i54') 


SOUND. 


105 


Fig.  92. 


MUSICAL  SOUNDS. 

[55.  Difference  between  Noise  and  Musical  Sounds. 
-In  Figure  92,  we  have  an  instrument  called  the  gyro- 
scope, consisting  mainly  of  a  heavy  brass  ring  d  surround- 
ing a  disk  which  rests  upon 
a  steel  axis.  To  this  axis 
is  fastened  a  small  toothed 
wheel  W.  If  a  card  c  be 
held  against  the  edge  of  the 
wheel  when  it  is  spinning 
rapidly,  a  very  shrill  musi- 
cal sound  is  produced;  as 
the  speed  is  checked  some- 
what, the  sound  becomes 
less  shrill.  The  more  the 
speed  is  diminished,  the  less 
shrill  the  sound  becomes, 
until  finally  we  hear  the 
separate  taps  of  the  teeth 
against  the  card. 

We  see,  then,  that  "when 
the  taps  are  sufficiently 
frequent,  they  blend  so  as 
to  produce  one  continuous 
sound,  or  a  musical  sound,  as  it  is  called. 

In  this  experiment,  the  card  is  made  to  vibrate  by  strik- 
ing the  teeth  of  the  wheel ;  and,  as  the  teeth  are  at  equal 
distances,  the  vibrations  follow  one  another  at  equal  inter- 
vals. A  musical  sound,  then,  is  one  in  which  the  vibra- 
tions recur  at  regular  intervals.  If  they  do  not  recur 
at  regular  intervals,  the  sound  is  called  a  noise. 

156.  The  Pitch  of  Musical  Sounds.  —  We  have  seen 
that,  the  faster  the  wheel  turns,  and  the  more  rapid  the 
vibrations  of  the  card,  the  shriller  is  the  sound,  or  the 


IO6  SOUND. 

higher  its  pitch.  Hence  the  pitch  of  musical  sounds 
depends  on  the  rapidity  of  the  vibrations. 

In  musical  sounds,  as  in  all  other  sounds,  the  loudness 
depends  upon  the  amplitude  of  the  vibrations. 

157.  The  Tuning-Fork.  —  The  tuning-fork  (Figure 
93)  consists  of  a  bar  of  steel  bent  into  the  form  of  the 

Fig.  93- 


letter  U,  and  attached  to  a  standard.  A  B  is  a  wooden 
case  open  at  both  ends,  by  which  the  intensity  of  the 
sound  produced  by  the  fork  is  increased.  The  fork  may 
be  set  vibrating  by  striking  it,  or  by  drawing  a  violin 
bow  across  it.  The  elasticity  of  the  steel  causes  the 
prongs  to  vibrate  regularly,  and  thus  to  give  out  a 
musical  sound. 

158.  The  Siren.  —  The  siren  (Figure  94)  is  an  in- 
strument for  producing  musical  sounds,  and  at  the 
same  time  registering  the  number  of  vibrations.  The 
disk  d  e  is  pierced  with  holes,  and  is  made  to  rotate  by 
blowing  into  the  tube  t.  As  it  rotates,  the  holes  are 
alternately  opened  and  closed,  so  that  the  air  escapes 
from  the  cylinder  in  a  regular  succession  of  puffs,  giving 
rise  to  vibrations,  which  produce  a  musical  sound. 

The   number  of  times  the  disk  rotates  is   registered 


SOUND. 


107 


by  the  apparatus  shown  in  the  upper  part  of  the  figure. 
On  the  axis  of  the  disk  is  an  endless  screw  s  (132), 
which  carries  a  pair  of  toothed  wheels.  These  are  con- 
nected with  pointers 
moving  over  dial- 
plates  on  the  front  of 
the  instrument.  The 
stops  m,  n,  o,  p,  are 
used  to  open  or  close 
;the  different  sets  of 
holes. 

159.  The  Rate  at 
'which     a     Sounding 
Body  vibrates  may  be 
determined  by  means 
of  the  Siren.  —  If  we 
force  air  into  the  siren 
with    a    bellows,    the 
disk  is  made  to  rotate 
faster  and  faster,  and 
the  pitch  of  the  sound 
produced  rises   higher 
and    higher,     as    the 
force  of  the  blast  in- 
creases.    In  this  way, 
the  siren  may  be  made 
to  produce  a  sound  of 
the  same  pitch  as  that 
of  a    tuning-fork,  or 
of  any  other  sounding 

body ;  and,  by  means  of  the  registering  apparatus, 
the  number  of  vibrations  in  a  second  may  be  ascer- 
tained. 

1 60.  The  Octave.  —  A  sound  is  the  octave  of  another 
when  it  is  produced  by  vibrations  twice  as  rapid. 


IOS  SOUND. 

161.  The  Sonometer.  —  The  sonometer  (sound  meas- 
urer}, shown  in  Figure  95,  consists  of  the  sounding-board 
M  N,  above  which  the  string  B  B1  is  stretched  upon 

Fig.  95- 


two  movable  bridges  by  means  of  the  weight  W.  It  is 
used  to  illustrate  the  laws  of  the  vibrations  of  strings. 

\62.  The  Rapidity  with  which  a  String  vibrates  is 
inversely  as  its  Length.  —  Cause  the  string  B  B1  to 
vibrate  by  pulling  it  to  one  side,  or  drawing  a  bow 
across  it,  and  notice  the  pitch  of  the  sound.  Place  one 
of  the  movable  bridges  at  the  centre  of  the  string,  so  as 
to  divide  it  into  two  equal  parts,  and  cause  either  part 
to  vibrate.  The  sound  will  be  the  octave  (160)  of  the 
one  given  out  by  the  whole  string.  The  half  of  a  string, 
then,  vibrates  twice  as  fast  as  the  whole  string,  when  the 
tension  (or  the  tightness  with  which  it  is  stretched)  re- 
mains the  same.  In  the  same  way,  it  can  be  proved 
that  one-third  of  a  string  vibrates  thrice  as  fast  as  the 
whole  ;  and  so  on.  While  the  string  is  equally  stretched, 
the  rapidity  of  its  vibrations  is  inversely  as  its  length. 

163.  The  formation  of  Nodes. — If  we  hold  a  feather 
against  the  centre  of  the  wire  of  the  sonometer  (Figure 


SOUND. 


109 


96),  and  draw  a  bow  across  one-half  of  it,  we  get  the 
octave  of  the  note  given  by  the  whole  string,  showing 
that  one-half  vibrates  by  itself.  If  now  a  little  rider  of 

Fig.  96. 


red  paper  be  placed  across  the  middle  of  one  part  of  the 
string,  and  the  other  part  be  made  to  vibrate  while  the 
feather  is  still  held  at  the  centre,  the  rider  is  thrown  off; 
showing  that  both  halves  of  the  string  vibrate.  These 
vibrating  halves  are  separated  by  a  node,  or  stationary 
point,  formed  where  the  feather  touches  the  string. 

Hold  now  the  feather  one-third  of  the  way  from  the 
end  of  the  wire  (Figure  97),  and  place  a  blue  rider  on 


Fig.  97- 


the  longer  portion  of  the  wire,  so  as  to  divide  it  into 
two  halves,  and  red  ones  on  the  middle  of  these  halves. 
Now  draw  the  bow  across  the  shorter  portion  of  the 


no 


SOUND. 


wire,  and  the  blue  rider  will  remain  at  rest  and  the 
others  be  thrown  off,  as  shown  in  the  figure ;  showing 
that  the  longer  portion  of  the  wire  has  been  divided 
into  two  vibrating  parts  separated  by  a  node. 

In  the  same  way,  the  wire  may  be  divided  into  four, 
five,  or  any  number  of  vibrating  parts  separated  by  nodes. 

164.  Formation  of  Nodes  in  Vibrating  Plates.  —  In 
Figure  98,  we  have  a  metallic  plate  supported  at  its 

Fig.  98- 


centre.  If  fine  sand  be  sprinkled  over  the  plate,  and  a 
bow  be  drawn  across  the  middle  of  one  edge  while  the 
thumb  and  finger  are  held  against  the  opposite  edge,  the 
sand  instantly  collects  into  lines,  as  seen  in  the  figure ; 
showing  that  the  vibrating  plate  is  at  rest  along  these 
lines.  The  sand  has  all  been  tossed  away  from  the 
vibrating  portions  between  the  lines. 

A.  vibrating  plate  may,  then,  be  broken  up  into 
different  vibrating  parts ;  and  the  lines  which  separate 
these  parts  are  called  nodal  lines. 

By  holding  the  thumb  and  finger  against  different 
parts  of  the  plate,  a  great  variety  of  nodal  lines  may 
be  obtained,  all  of  which  may  be  made  visible  by  means 


SOUND. 


Ill 


of  sand,  as  in  the  above  experiment, 
nodal  forms  are  shown  in  Figure  99. 

Fig.  99- 


Some  of  these 


Nodes  may  be  formed  in  a  similar  way  in  bells,  and 
in  all  other  sounding  bodies. 

165.  Overtones  or  Harmonics.  —  It  is  found  that,  even 
when  a  sounding  body  is  made  to  vibrate  as  a  whole,  it 
always  does  at  the  same  time  vibrate  in  parts ;  so  that 
a  vibrating  body  never  gives  out  a  simple  tone.  The 
tone  given  out  by  a  string,  or  other  body,  as  a  whole,  is 
called  its  fundamental  note;  the  higher  tones  produced 
by  the  vibrations  of  the  parts  are  called  harmonics, 
or  overtones.  The  tone  produced  by  the  halves  of  a 
string  is  called  the  first  harmonic;  that  produced  by 
the  thirds  of  a  string,  the  second  harmonic ;  and 
so  on. 


112 


SOUND. 


Fig.  100. 


1 66.  Quality.  —  In    every   vibrating   string,    a    great 
number  of   these   higher  tones    are    produced,   which, 
mingling  with  the  fundamental  tone,  give  rise  to  what 
is  called  the  quality  of  the  sound.     It  is  this  union  of 
high  and  low  tones  which  enables  us  to  distinguish  one 
musical  instrument  from  another.     A  flute  and  a  violin, 
though   tuned   to   the   same    fundamental    note,    do    not 
give  the   same  sound.      The  overtones  of  the  one  are 
different   from   those   of  the   other;    and   the    mixtures 
formed  by  these  and  the  fundamental  note  are  therefore 
different. 

167.  —  Musical    Sounds    are    transmitted   through 
Liquids   and   Solids.  —  In   Figure    100,    M  is   a   long 

tube  filled  with  water, 
which  is  placed  between 
the  tuning-fork  1?  and 
the  sounding-box  A  JB. 
If  the  fork  be  set  vibrat- 
ing in  the  air  away  from 
the  tube,  it  can  scarcely 
be  heard ;  but  if  the  foot 
of  it  be  placed  upon  the 
"water  in  the  tube,  it  can 
be  heard  as  distinctly  as 
wljen  it  is  placed  upon 
the  sounding-box.  In  both 
cases,  the  box  is  the  real 
sounding  body,  and  is  set 
vibrating  by  means  of  the 
tuning-fork.  Musical  vi- 
brations, then,  are  trans- 
mitted through  the  water 

in  the  tube.      Similar  experiments  prove  that  they  are 

transmitted  through  all  liquids. 

By  using  a  rod  of  wood  in  place  of  the  tube,  it  may 


SOUND.  1 13 

be   shown  that  musical  sound  is  transmitted  through 
solids  as  well  as  through  liquids  and  gases.* 

1 68.  Sympathetic  Vibrations.  —  Place  two  tuning- 
"'  forks  which  sound  the  same  note,  mounted  on  their 
boxes,  upon  the  table,  18  inches  apart,  and  draw  the  bow 
vigorously  across  one  of  them.  If  now  we  stop  the 
agitated  fork,  the  sound  is  weakened,  but  by  no  means 
quenched.  The  vibrations  conveyed  through  the  air 
and  through  the  'wood  have  been  taken  up  by  the  un- 
touched fork,  and  it  is  this  fork  which  we  now  hear. 
Attach  a  bit  of  wax  to  one  of  the  forks,  and  sound  it 
again  ;  the  very  slight  change  in  the  rate  of  vibration  has 
destroyed  the  sympathy  between  the  two  forks,  and  no 
response  is  now  possible.  Remove  the  wax,  and  the  un- 
touched fork  responds  as  before.  \ 

*  An  experiment  first  tried  by  Wheatstone  and  repeated  by 
Tyndall  is  very  striking.  A  piano  was  placed  in  a  room  under- 
neath the  lecture-room,  separated  from  the  latter  by  two  floors. 
Through  the  two  floors  passed  a  tin  tube  2i  inches  in  diameter, 
with  a  wooden  rod  inside  of  it,  the  end  of  which  projected  into 
the  lecture-room.  The  rod  was  clasped  by  India-rubber  bands 
which  completely  closed  the  tube.  The  lower  end  of  the  rod 
rested  upon  the  sounding-board  of  the  piano.  The  piano  was 
played,  and  no  sound  was  heard  in  the  lecture-room ;  but  when 
a  violin  was  placed  against  the  end  of  the  rod,  it  became  musi- 
cal, not  with  the  vibrations  of  its  own  strings,  but  with  those  of 
the  piano.  On  taking  away  the  violin,  the  music  ceased;  but 
when  a  guitar  was  put  in  its  place,  the  sounds  were  heard 
again;  and  also  when  a  sounding-box  was  substituted  for 
the  guitar.  The  end  of  the  rod  was  then  placed  against  the 
sounding-board  of  a  harp,  and  every  note  of  the  piano  was 
reproduced  as  before. 

An  ordinary  music-box  may  be  used  instead  of  the  piano  in 
this  experiment. 

f  The  vibrations  may  be  communicated  through  the  air  alone. 
Every  one  knows  that  a  piano-string  is  sometimes  set  vibrating 
when  the  note  of  the  string  is  sounded  by  the  voice  or  a  flute, 
even  at  the  other  end  of  the  room. 


114  SOUND. 

When  a  body  is  thus  thrown  into  vibration  by  its 
neighbor,  its  vibrations  are  said  to  be  sympathetic. 

169.  Two  Sounds  may  Interfere  so  as  to  destroy  each 
other  and  produce  Silence.  —  There  are  certain  posi- 
tions in  which  the  sound  of  one  prong  of  a  tuning-fork  is 
wholly  destroyed  by  that  of  the  other.  These  positions 
are  easily  found  by  making  the  fork  vibrate,  and  then 
turning  it  round  before  the  ear.  When  the  back  or  the 
side  of  a  prong  is  parallel  to  the  ear,  the  sound  is  heard ; 
when  the  corner  of  a  prong  is  held  toward  the  ear,  the 
sound  is  utterly  destroyed. 

This  case  of  interference,  as  it  is  called,  may  be  ren- 
dered more  striking  by  means  of  a  resonant  jar  (180). 
In  Figure  101,  the  jar  is  of  such  a  length  as  to  resound 

Fig.  101. 


powerfully  to  the  fork.  Rotate  the  fork  above  the  mouth 
of  the  jar.  When  the  back  or  sides  of  the  prongs  face 
the  jar,  a  loud  sound  is  obtained ;  but  when  the  corners 
of  the  fork  face  the  jar,  there  is  no  sound. 

When  the  corner  of  the  fork  is  over  the  jar,  slide  a 
pasteboard  tube  over  one  prong  so  as  to  cut  off  its  vibra- 
tions, and  the  jar  begins  to  resound. 


SOUND.  115 

1 70.  Beats.  —  If  two  tuning-forks  which  vibrate  nearly 
at  the  same  rate  be  made  to  sound  together,  the  sound, 
instead  of  being  continuous,  rises  and  falls  in  quick 
succession,  producing  what  are  called  beats. 

Beats  are  thus  produced  'whenever  two  musical  sounds 
of  nearly  the  same  pitch  are  uttered  together,  and  the 
number  of  beats  per  second  is  always  equal  to  the  dif- 
ference between  the  two  rates  of  vibration. 

iji.  Combination  of  Musical  Sounds.  —  Take  two 
tuning-forks,  each  of  which  gives  256  vibrations  in  a 
second,  and  set  them  vibrating.  The  two  musical  sounds 
flow  together  in  a  perfectly  blended  stream,  and  produce 
what  is  called  unison.  In  this  case,  the  ratio  of  the 
vibrations  is  I  :  I. 

Take  now  two  forks,  one  of  which  makes  256  vibra- 
tions a  second,  and  the  other  512.  For  every  vibration 
sent  to  the  ear  by  the  one  fork,  two  vibrations  are  sent 
by  the  other,  and  the  two  notes  blend  harmoniously. 

This  combination,  as  we  have  seen,  is  called  an  octave 
(160)  ;  and  the  ratio  of  the  vibrations  is  1:2. 

Take  another  pair  of  forks,  which  give  256  and  384 
vibrations  in  a  second.  The  combination  of  the  two 
sounds  is  very  pleasing  to  the  ear,  but  the  consonance  is 
hardly  so  perfect  as  in  the  case  of  the  octave.  The  ratio 
of  the  vibrations  is  2  :  3.  This  is  the  most  pleasing  com- 
bination next  to  the  octave,  and  is  called  a.  fifth. 

If  we  take  two  forks  whose  vibrations  are  in  the  ratio 
3  : 4,  the  interval  is  called  a  fourth.  This  combination 
is  still  agreeable,  but  not  quite  so  agreeable  as  the  fifth. 

Thus,  then,  with  perfect  unison  the  ratio  of  the 
vibrations  is  i  :  i  ;  with  a  note  and  its  octave  it  is  i  :  2  ; 
with  a  note  and  its  fifth  it  is  2  :  3  ;  and  with  a  note  and 
its  fourth  it  is  3  :  4.  The  combination  of  two  notes  is 
the  more  pleasing  to  the  ear,  the  smaller  the  two  num- 
bers which  express  the  ratio  of  their  vibrations. 


Il6  SOUND. 

Take  now  two  forks  whose  rates  of  vibration  are  in  the 
ratio  4 :  5,  or  a  major  third  apart ;  the  harmony  is  less 
perfect  than  in  any  of  the  cases  which  we  have  examined. 
With  the  ratio  5  :  6,  or  that  of  a  minor  third,  it  is  usually 
less  perfect  still ;  and  we  now  approach  a  limit  beyond 
which  a  musical  ear  will  not  tolerate  the  combination  of 
two  sounds.  If,  for  example,  we  sound  together  two  forks 
whose  vibrations  are  in  the  ratio  of  13  :  14,  their  combi- 
nation is  altogether  discordant. 

An  agreeable  combination  of  two  notes  is  called  a 
chord;  a  disagreeable  one,  a  discord. 


SUMMARY. 

When  the  vibrations  of  a  sounding  body  take  place  at 
regular  intervals  and  often  enough,  they  give  rise  to  a 
musical  sound.  In  a  noise,  the  vibrations  follow  one  an- 
other at  irregular  intervals.  (155.) 

The  pitch  of  the  sound  increases  with  the  rapidity  of 
the  vibrations.  By  means  of  the  siren,  we  may  ascer- 
tain the  number  of  vibrations  answering  to  any  given 
pitch.  (156,  159.) 

Strings,  plates,  and  all  sounding  bodies  may  vibrate  in 
parts  separated  by  nodes.  (163,  164.) 

Sounding  bodies  always  vihrate  in  parts,  giving  rise  to 
overtones  or  harmonics;  and  the  blending  of  these  vibra- 
tions gives  to  the  sound  its  quality.  (165,  166.) 

Musical  sounds  are  transmitted  through  solids,  liquids, 
and  gases.  (167.) 

A  vibrating  body  may  throw  another  body  into  sympa- 
thetic vibration.  (168.) 

Two  musical  sounds  may  interfere  so  as  to  produce 
silence.  (169.) 


SOUND.  117 

When  two  musical  sounds  of  nearly  the  same  pitch  are 
sounded  together,  beats  are  produced.  (170.) 

When  the  combination  of  two  notes  is  agreeable,  they 
form  a  chord;  when  it  is  disagreeable,  a  discord.  The 
simpler  the  ratio  of  the  vibrations  of  two  notes,  the 
more  agreeable  the  chord  which  they  form.  (171.) 


MUSICAL    INSTRUMENTS. 

STRINGED  INSTRUMENTS. 

\  J 

A  172.  Stringed  Instruments.  —  In  stringed  instru- 
ments the  sounds  are  produced  by  the  vibrations  of 
strings  or  wires. 

173.  Sounding- Boards.  —  Some  kind  of  a  sounding- 
board  is  necessary  in  all  stringed  instruments. 

It  is  not  the  chords  of  a  piano,  or  harp,  or  violin, 
that  throw  the  air  into  sonorous  vibrations.  It  is  the 
large  surfaces  connected  with  the  strings,  and  the  air 
enclosed  by  these  surfaces.  The  merit  of  such  instru- 
ments depends  mainly  upon  the  quality  and  arrange- 
ment of  their  sounding-boards. 

1 74.  Laws  of  the  Vibration  of  Strings.  —  The  first 
law  of  the  vibration  of  strings  has  already  been  found 
(162),  and  is  stated  thus:    The  rapidity  of  the  vibra- 
tions is  inversely  as  the  length  of  the  string. 

175.  The  Rapidity  with   which  a   String  vibrates 
varies    as    the    Square    Root    of   the    Weight    which 
stretches  it.  —  If  a  string  be  stretched  on  the  sonome- 
ter  (161)   with   a  weight  of  one   pound   and   made   to 
vibrate,   a  note  of  a  certain  pitch  is  obtained.     If  the 
weight  be  made  four  pounds,  the  pitch  will  be  raised 


Il8  SOUND. 

an  octave;  if  sixteen  pounds,  it  will  be  raised  another 
octave ;  and  so  on.  The  rapidity  of  the  vibrations, 
then,  varies  as  the  square  root  of  the  weight  by  which 
the  string  is  stretched. 

176.  The  Rapidity  with  which  a   String  vibrates 
varies  inversely  as  its  Thickness.  —  If  strings  of  the 
same  material,  but  of  different  thickness,  be  stretched 
by  equal  weights,  the  thicker  strings  will  give  the  lower 
notes.     If  one  string  is  just  twice  as  thick  as  another, 
its  note  will  be  an  octave  lower.     Other  things  being 
equal,  the  rapidity  of  the  vibrations  of  a  string  varies 
inversely  as  its  thickness. 

177.  The  Rapidity  with   which  a   String  vibrates 
is  inversely  as  the  Square  Root  of  its  Density.  —  If  a 
platinum  and  an  iron  wire  of  the  same  length  and  thick- 
ness be  stretched  by  equal  weights,  they  will  not  give  notes 
of  the  same  pitch.    It  is  found  that  the  pitch  of  the  sound 
rises  as  the  square  root  of  the  density  diminishes. 

The  last  two  laws,  taken  together,  may  be  stated  thus : 
The  rapidity  with  which  strings  vibrate  is  inversely 
proportional  to  the  square  root  of  their  weight. 

In  one  class  of  stringed  instruments,  like  the  violin, 
violoncello,  and  guitar,  notes  of  a  great  variety  of 
pitch  are  obtained  from  a  few  strings  by  Jingering 
the  strings,  so  as  to  change  their  length.  In  another 
class,  like  the  harp  and  piano-forte,  many  strings  are 
used,  varying  in  length  and  thickness,  each  of  which 
gives  but  one  note. 


SUMMARY. 

Musical  sounds  may  be  produced  by  the  vibrations  of 
strings,  but  a  sounding-board  is  necessary  to  make  them 
audible.  (172,  I73-) 


SOUND.  119 

The  laws  of  vibrating  strings  are  three  in  number  :  — 

(1)  The  rapidity  with  which  a  string  vibrates  varies 
inversely  as  its  length. 

(2)  The  rapidity  with  which  a  string  vibrates  varies 
as  the  square  root  of  the  weight  which  stretches  it. 

(3)  The  rapidity  with  which  a  string  vibrates  is  in- 
versely as  the  square  Joot  of  its  weight. 

In  some  stringed  instruments  many  notes  are  produced 
by  a  few  strings  ;  in  others,  there  are  as  many  strings  as 
there  are  notes  given.  (174-177*) 


WIND   INSTRUMENTS. 

178.  The  Longitudinal  Vibrations  of  Rods  free  at 
one  End.  —  A  smooth  wooden   or   metallic   rod,  with 
one  of  its  ends  fixed  in  a  vice,  yields  a  musical  note 
when  rubbed  with  resined  leather.     The  rod  lengthens 
and  shortens  in  quick  succession,  or,  in  other  words,  is 
thrown  into  longitudinal  vibration.     The  pitch  of  the 
note    increases    as    the    length    of   the    rod    diminishes. 
Figure   102   shows   a   musical    instrument  whose   notes 
are    produced   by   the    longitudinal  Fig.  102. 
vibrations   of  wooden   rods   of  dif- 
ferent lengths. 

1 79.  Longitudinal  Vibrations  of 
Rods  free  at  both  Ends.  —  Clasp 
a  long  glass  tube  at  its  centre  with 
one  hand,  and  rub  a  wet  cloth  over 
one   of  its   halves  with   the   other. 
A  musical  sound  is  produced.     A 
solid  glass  rod  of  the  same  length 
will   give   the  same  note.     In  this 
case,  the  centre  of  the  tube,  or  rod, 
is  a  node  (163),  and  the  two  halves 


120 


SOUND. 


lengthen  and  shorten  in  quick  succession.      In  Figure 
103,  a  b  is  a  brass  rod  held  at  its  centre  by  the  clamp 


Fig.  103. 


s;   and   an   ivory  ball   hung  by  two   strings  from   the 
points  m  and  n  rests  against  the  end  b  of  the  rod.     On 
drawing  a  piece  of  resined  leather  gently  over  the  rod 
Fig.  104.  near  # ,  we  throw  it  into  longitudinal 

vibrations.  The  centre  s  is  at  rest,  but 
the  motion  of  the  ivory  ball  shows  that 
the  end  b  is  in  a  state  of  tremor.  Rub 
the  rod  more  briskly,  and  its  vibrations 
become  more  intense,  and  the  ivory 
ball  is  thrown  off  violently  whenever 
it  comes  in  contact  with  the  end  of 
the  rod. 

If  a  long  glass  tube  be  held  at  the 
centre,  and  one-half  of  it  be  rubbed 
briskly  with  a  wet  cloth,  the  strain 
upon  the  glass,  caused  by  the  longi- 
tudinal vibrations,  may  be  sufficient  to 
shiver  the  other  end,  as  shown  in  Fig- 
ure 104. 

1 80.  Resonance.  —  When  a  tuning- 
fork  is  detached  from  the  sounding-box,  and  made  to 
vibrate,  it  can  hardly  be  heard.  Let,  now,  the  fork  be 


SOUND. 


121 


Fig.  105. 


held  over  a  glass  jar  AB  (Figure  105),  some  18  inches 
deep,  and  the  sound  is  still  very  faint.  Keep  the  fork 
in  this  position,  and 
pour  water  with  the 
least  possible  noise  into 
the  jar.  As  the  col- 
umn of  air  under  the 
fork  becomes  shorter, 
the  sound  becomes 
louder ;  and  when  the 
water  has  reached  a 
certain  level,  it  bursts 
forth  with  great  power. 
Continue  to  pour  water 
into  the  jar,  and  the 
sound  becomes  weaker 
and  weaker,  until  it  is 
as  faint  as  at  first. 
Pour  the  water  care- 
fully out,  and  we  reach 
a  point  where  the  sound 
is  reinforced  again.  In  this  way,  we  find  that  there 
is  one  particular  length  of  the  column  of  air  'which 
causes  the  fork  above  it  to  give  the  loudest  possible 
sound.  This  reinforcement  of  sound  is  called  reso- 
nance.* 

The  columns  become  shorter  as  the  forks  vibrate 
faster. 

*  "Most  travellers  in  Switzerland  have  noticed  the  deafening 
sound  produced  by  the  fall  of  the  Reuss  at  the  Devil's  Bridge. 
The  noise  of  the  fall  is  raised  by  resonance  to  the  intensity 
of  thunder.  The  sound  heard  when  a  hollow  shell  is  placed 
close  to  the  ear  is  a  case  of  resonance.  Children  think  they 
hear  in  it  the  sound  of  the  sea.  The  noise  is  really  due  to 
the  reinforcement  of  the  feeble  sounds  with  which  even  the 
stillest  air  is  pervaded.  The  channel  of  the  ear  itself  is  also 


122  SOUND. 

181.  A   Column  of  Air  may  be  made  to  vibrate  by 
blowing  across  the  End  of  a  Tube.  —  Select  two  jars, 
and  two  tuning-forks  to  which  they  will  resound.     Make 
both   forks  vibrate,    and    hold    them   both    over   one   of 
the  jars.     Only  one  of  them  is  heard.     Hold  them  both 
over  the  other  jar,  and  the  other  fork  alone  is  heard. 
If  twenty  forks  were  held  over  either  of  these  jars,  it 
would  select  and  reinforce  the  sound  of  the  one  to  which 
it  naturally  resounds. 

Blow,  now,  across  the  open  mouth  of  this  same  jar, 
or  across  the  mouth  of  a  glass  tube  of  the  same  length 
as  the  jar,  and  f  of  an  inch  in  diameter  (Figure  106). 
Fig.  106.      A  fluttering  of  the  air,  a  mere  medley  of  vibra- 
tions,  is  thus  produced  at  the  mouth  of  the 
tube.     The  tube  selects  the  set  of  vibrations, 
or  pulse,  to  which   it   can   resound,  and   re- 
inforces it  so  that  it  becomes  a  musical  sound. 
The  sound  is  the  same  as  that  produced  by 
the    proper    tuning-fork    held    over    the   tube. 
The  column  of  air  in  the  tube  has,  in  fact, 
made    its   own   tuning-fork ;    for  it   has  made 
the  air  blown  across  the  tube  vibrate  in  uni- 
son with  itself. 

On  blowing  across  the  mouth  of  a  tube  of 
any  length,  a  musical  sound  is  produced  ex- 
actly like  that  obtained  when  the  proper  tuning-fork 
is  held  over  the  tube. 

182.  The  Rate  of  Vibration  of  a  Column  of  Air  in 
a  Tube  is  inversely  proportional  to  its  Length.  —  Take 
three  tubes,  6,  3,  and  ij  inches  long,  and  blow  gently 

a  resonant  cavity.  When  a  poker  is  held  by  two  strings,  and 
when  the  fingers  of  the  hands  holding  the  poker  are  thrust 
into  the  ears,  on  striking  the  poker  against  a  piece  of  wood 
a  sound  is  heard  *s  deep  and  sonorous  as  that  of  a  cathedral 
bell."  —  Tyndatt. 


SOUND. 


I23 


across  the  mouth  of  each,  so  as  to  bring  out  its  funda- 
mental note  (165).  The  note  of  the  3-inch  tube  will 
be  the  octave  of  the  note  of  the  6-inch  tube,  and  that 
of  the  i^-inch  tube  the  octave  of  that  of  the  3-inch 
tube.  In  other  words,  the  rate  of  vibration  is  in- 
versely proportional  to  the  length  of  the  tube. 

183.  Vibrations  in  Open  Tubes.  —  The  tubes  which 
have  been  used  thus  far  have  been  closed  at  one  end. 
Such  tubes  are  called  stopped  tubes.  We  will  next 
examine  the  vibrations  of  tubes  open  at  both  ends,  or 
open  tubes.  If  we  take  a  stopped  tube  and  an  open 
tube  of  the  same  length,  and  blow  gently  across  the 
mouth  of  each  so  as  to  get  its  fundamental  note,  we 
find  the  note  of  the  latter  an  octave  higher  than  that  of 

Fig.  107.  Fig.  108. 


I  24 


SOUND. 


the  former.  An  open  tube  always  yields  the  octave 
of  the  note  given  by  a  stopped  tube  of  the  same  length. 
184.  Organ- Pipes.  —  Organ-pipes  are  nothing  more 
than  resonant  tubes.  There  are  various  ways  of  agitat- 
ing the  air  at  the  mouth  of  such  tubes,  so  as  to  set 
the  columns  of  air  within  them  into  vibrations.  In  one 
kind  of  organ-pipes,  this  is  done  by  blowing  a  thin 
sheet  of  air  against  a  sharp  edge.  This  produces  a 
flutter,  some  pulse  (181)  of  which  is  converted  into  a 
musical  sound  by  the  resonance  of  the  air  in  the  tube. 
Figures  107  and  108  represent  open  organ-pipes.  The 
air  passes  from  the  bellows  through  the  tube  JP  into  a 
chamber,  which  is  closed  at  the  top  except  the  narrow 
slit  t.  The  air  compressed  in  the  chamber  passes 
through  this  slit  in  a  thin  sheet,  which  breaks  against 
the  sharp  edge  «,  and  there  produces  a  flutter.  The 
space  between  the  edge  a  and  the  slit  below  is  called 
the  mouth  of  the  pipe. 

In   a   stopped  organ-pipe,  the   upper   end    is   closed. 

Instead  of  producing  a  flutter 
at  the  mouth  of  the  pipe 
by  a  blast  of  air,  \ye  may 
get  the  same  effect  by  hold- 
Ing  at  the  mouth  of  the  pipe 
(Figure  109)  a  tuning-fork 
whose  vibrations  are  in  uni- 
son with  those  of  the  pipe. 
Select  several  pipes  of  differ- 
ent lengths,  and  tuning-forks 
in  unison  with  each.  Begin- 
ning with  the  longest  pipe, 
make  the  fork  of  lowest  pitch 
vibrate  near  its  mouth.  The 
pipe  speaks  loudly.  Blow  in- 
to the  same  pipe ;  its  tone  is 


SOUND. 


'25 


exactly  the  same  as  when  the  fork  was  held  at  its  mouth. 
Try  each  pipe  in  the  same  way,  and  the  note  which 
each  gives  when  blown  into  is  exactly  that  given  when 
the  proper  fork  is  at  its  mouth.  If  all  the  forks  are  held 
at  the  same  time  at  the  mouth  of  any  one  of  the  pipes, 
the  pipe  will  select  and  reinforce  the  sound  of  but  one. 
So  also  the  current  of  air  striking  against  the  sharp 
upper  edge  of  the  mouth  of  the  pipe  gives  rise  to  a 
great  variety  of  pulses,  from  which  the  pipe  selects  and 
reinforces  but  one. 

185.  Reed  Pipes.  —  A  column  of  air  may  be  made  to 
vibrate  by  means  of  a  spring  of  metal,  or  wood,  called 
a  reed.     The  metal  reed  commonly  used  in  organ-pipes 
is  shown  in  Figure  no.     It  consists  of  a  long  and  flexi- 
ng, no. 


ble  strip  of  metal,  V  V,  placed  in  an  opening  through 
which  the  air  enters  the  pipe.  As  soon  as  the  air 
begins  to  enter  the  pipe,  the  force  of  the  blast  bends 
down  the  spring  of  the  reed  so  as  to  close  the  opening. 
The  elasticity  of  the  reed  causes  it  to  fly  back  at  once, 
so  as  to  open  the  pipe  and  allow  the  air  to  enter  again. 
It  thus  breaks  up  the  current  of  air  into  a  regular  suc- 
cession of  little  puffs. 

The  action  of  the  reed  may  be  illustrated  by  a  com- 
mon straw.  With  a  penknife  raise  a  strip  of  the  straw 
near  a  knot,  as  shown  at  r  r1  in  Figure  in.  This  strip 


126  SOUND. 

serves  as  a  reed,  and  the  straw  as  a  pipe.     Blow  into 
it,  and  it  gives  a  musical  note. 

Fig.  in. 


s 


In  the  horn,  trumpet,  and  similar  instruments,  the  lips 
of  the  player  take  the  place  of  the  reed. 

1 86.  Two  Classes  of  Wind  Instruments.  —  In  one 
class  of  wind  instruments,  as  the  flute  and  fife,  a  single 
column  of  air  is  made  to  give  a  great  number  of  notes, 
the  length  of  the  column  being  varied  by  keys.  In  an- 
other class,  as  the  organ,  there  is  a  pipe  for  every  note. 


SUMMARY. 

Longitudinal  vibrations  may  be  illustrated  by  means  of 
rods  free  at  one  end  or  at  both  ends.  (178,  179.) 

The  sound  of  a  tuning-fork  is  reinforced  when  it  vi- 
brates over  the  mouth  of  a  jar  of  air  of  a  certain  depth. 
This  reinforcement  of  sound  is  called  resonance.  (180.) 

Jars  and  tubes  may  be  made  resonant  by  blowing  across 
their  open  mouths,  and  give  the  same  note  as  when  made 
to  resound  by  a  tuning-fork.  The  shorter  the  column  of 
air,  the  faster  it  vibrates.  (181,  182.) 

An  open  tube  gives  a  note  which  is  the  octave  of  a 
closed  tube  of  the  same  length.  (183.) 

Organ-pipes  are  resonant  tubes.  When  open  at  both 
ends,  they  are  called  open  pipes ;  when  closed  at  one  end, 
stopped  pipes. 

One  kind  of  organ-pipe  is  made  to  resound  by  blowing 
a  thin  sheet  of  air  against  a  sharp  edge  at  its  mouth. 

(i84.) 

A  column  of  air  may  be  made  to  vibrate  by  means  of 
a  reed.  The  trumpet  and  many  other  wind  instruments 
are  reed-pipes.  (185.) 


SOUND. 


SOUNDING   FLAMES. 


127 


Fig.  112. 


187.  Friction  is  always  Rhyth- 
mic. —  When  we  draw  a  bow  across 
a  string,  or  rub  a  wet  ringer  round 
the  edge  of  a  glass,  a  musical  sound 
is  produced,  showing  that  the  fric- 
tion has  been  broken  up  into  rhyth- 
mic pulses.  Close  the  lower  end  of 
the  tube  A  B  (Figure  112)  with  a 
metallic  plate,  pierced  by  a  round 
hole  whose  diameter  is  equal  to  the 
thickness  of  the  plate.  Plug  the 
hole,  and  fill  the  tube  with  water. 
Remove  the  plug,  and,  as  the  water  - 
sinks  in  the  tube,  a  very  sweet  musi- 
cal note  is  given  out  by  the  liquid 
column.  This  note  is  due  to  the  in- 
termittent flow  of  the  water  through 
the  hole,  by  which  the  column  above 
is  thrown  into  vibrations.  The  same 
intermittence  is  observed  in  the  dense 
smoke  which  rolls  in  rhythmic  rings 
from  the  funnel  of  a  steamboat.  A  rifle-ball  sings  as 
it  passes  through  the  air.  u  The  whispering  pines  "  owe 
their  music  to  the  rubbing  of  the  wind  against  their 
branches  and  foliage.  The  whistling  of  the  wind  is 
also  produced  by  the  rhythmic  friction  of  the  air. 

If  we  blow  gently  against  a  candle-flame,  the  fluttering 
noise  announces  a  rhythmic  action.  We  have  learned 
(184)  that  a  pipe  will  select  a  pulse  from  a  flutter,  and 
raise  it  by  resonance  to  a  musical  sound.  In  like  manner, 
the  noise  of  a  flame  may  be  converted  into  a  musical 
note.  This  is  done  by  enclosing  the  flame  in  a  tube. 


128  SOUND. 

1 88.  Sensitive  Flames  within  Tubes.  —  Place  a  tube 
12  inches  long  over  a  small  gas-flame,  so  that  the  flame 
shall  be  about  an  inch  and  a  half  from  trie  bottom  of  the 
tube.  If  the  note  to  which  the  tube  would  resound  be 
sounded  at  some  distance,  the  flame  is  seen  to  tremble. 
Lower  the  tube,  so  that  the  flame  shall  be  about  three 
inches  from  the  bottom,  and  the  flame  begins  to  sing. 
Somewhere  between  these  two  points  we  may  find  a 
point  where  the  flame  will  burn  silently ;  but  if  it  be 
excited  by  the  voice,  it  will  sing,  and  keep  on  singing. 

Flames  which  are  thus  affected  by  musical  sounds  are 
called  sensitive  flames. 


SUMMARY. 

Friction  is  always  rhythmic. 

When  a  gas-flame  is  surrounded  by  a  tube,  the  air  in 
passing  over  it  is  made  to  vibrate,  and  musical  sounds 
are  produced.  A  silent  flame  within  a  tube  may  be 
made  to  sing  by  sounding  the  note  of  tfye  tube  near  it. 
(187,  188.) 

THE   HUMAN  VOICE  AND  EAR. 

189.  The  Organ  of  Voice  is  a  Reed  Instrument. — 
The  organ  of  voice  in  man  is  situated  at  the  top  of  the 
windpipe,  or  trathea,  which  is  the  tube  through  which 
the  air  is  blown  from  the  lungs.  A  pair  of  elastic 
bands,  called  the  vocal  chords,  stretched  across  the  top 
of  the  windpipe,  so  as  nearly  to  close  it,  form  a  double 
reed.  When  the  air  is  forced  from  the  lungs  through 
the  slit  between  these  chords,  they  are  made  to  vibrate. 
By  changes  in  their  tension,  their  rate  of  vibration  is 
varied,  and  the  sound  raised  or  lowered  in  pitch.  •  The 
cavity  of  the  mouth  and  nose  acts  as  a  resonant  tube. 


SOUND. 


I29 


The  action  of  the  vocal  chords  may  be  imitated  by 
Fig.  n3.  means  of  india-rubber  bands. 

If  the  open  end  of  a  glass 
tube  (Figure  113)  be  closed 
by  two  strips  of  india-rubber, 
leaving  a  slit  between  them, 
and  the  air  be  blown  through 
this  slit,  the  strips  are  thrown 
into  vibration,  and  a  musical 
sound  is  produced. 

190.    The  Human  Ear.  —  The   external  opening  of 

Fig.  114. 


the  ear  (Figure  1 14)  is  closed  at  the  bottom  by  a  mem- 
brane, called  the  tympanum.  Behind  this  is  the  cavity 
called  the  drum  of  the  ear.  This  is  separated  from 
the  space  between  it  and  the  brain  by  a  bony  partition, 
in  which  there  are  two  openings,  the  one  round  and 
the  other  oval.  These  also  are  closed  by  delicate  mem- 
branes. Across  the  cavity  of  the  drum  stretches  a  series 
of  four  little  bones :  the  first,  called  the  hammer,  is  at- 
tached to  the  tympanum ;  the  second,  called  the  anvil, 

9 


I3O  SOUND. 

is  connected  by  a  joint  with  the  hammer ;  a  third  little 
round  bone  connects  the  anvil  with  the  stirrup  bone, 
which  has  its  oval  base  planted  against  the  membrane 
of  the  oval  opening.  Behind  the  bony  partition  is  the 
labyrinth,  which  is  filled  with  water,  and  over  the  lining 
of  which  the  fibres  of'the  auditory  nerve  are  spread. 
The  tympanum  receives  the  vibrations  of  the  air,  and 
transmits  them  through  the  series  of  bones  to  the  mem- 
brane which  separates  the  drum  from  the  labyrinth ; 
and  thence  to  the  liquid  within  the  labyrinth,  which 
transmits  them  to  the  nerves.  The  nerve  transmits  the 
impression  to  the  brain  and  thus  to  the  mind. 

191.  The  Range  of  the  Human  Ear.  —  There 
must  be  at  least  16  vibrations  in  a  second,  in  order 
that  they  may  be  heard  as  a  continuous  sound  (155)  ; 
and  the  sound  ceases  to  be  audible  when  the  vibra- 
tions reach  38,000  in  a  second.  Starting  with  16  and 
multiplying  continually  by  2,  we  find  that  the  nth 
octave  will  have  32,768  vibrations.  Thus  the  entire 
range  of  the  human  ear  extends  to  about  1 1  octaves. 
The  practical  range  of  musical  sounds  is  about  7  oc- 
taves, or  from  40  to  4,000  vibrations  in  a  second. 


SUMMARY. 

The  organ  of  voice  in  man  is  a  reed  instrument,  the 
vocal  chords  forming  the  reed.  (189.) 

The  human  ear  consists  of  three  parts :  the  outer 
ear,  the  drum,  and  the  labyrinth.  The  sonorous  vibra- 
tions are  first  intercepted  by  the  tympanum,  then  trans- 
mitted to  the  fluid  in  the  labyrinth,  by  which  they  are 
communicated  to  the  auditory  nerve.  (190.) 

The  range  of  human  hearing  embraces  about  eleven 
octaves.  (191.) 


LIGHT 


PROPAGATION  OF  LIGHT. 


RADIATION,   REFLECTION,  AND  REFRACTION. 

192.  A  Luminous  Body  sends  out  Light  in  Every 
Direction. — A  body  in  which  light  is   developed  is 
called  a  luminous  body.     All  other  bodies  are  said  to  be 
non-himinous.     If  a  lighted  lamp  is  placed  in  the  middle 
of  a  room,  it  illumines  every  part  of  the  room  ;  showing 
that  the  light  proceeds  from  the  luminous  body  in  every 
direction. 

A  body  through  which  light  passes,  as  air  and  glass, 
is  called  transparent.     Other  bodies  are  called  opaque. 

193.  Light    travels    through     Space    in    Straight 
Lines.  —  If  a  room  be  darkened,   and  the   sunlight  be 
allowed  to  enter  through  a  small  hole  in  the  shutter,  it 
will   illumine   the  floating  particles  of  dust  in  the  air 

Fig.  115. 


through  which  it  passes,  so  that  we  can  trace  its  path ; 
and  in  every  case  we  find  that  it  moves  in  a  straight  line. 


I32 


LIGHT. 


An  opaque  body  placed  before  a  luminous  one  cuts  off 
the  light  from  the  space  behind  it,  producing  a  shadow* 

If  the  luminous  body  S  (Figure  115)  is  a  mere  point, 
the  body  J/will  cast  a  ivell-dejlned  shadow  G  If  upon 
the  screen  Pjg. 

If  the  luminous  body  S  L  (Figure  116)  is  not  a  mere 
point,  the  shadow  cast  by  M N  will  have  an  indistinct 

Fig.  116. 


outline.  The  dark  central  portion  G  //of  the  shadow 
is  called  the  umbra;  the  less  dark  outer  portion  is  called 
the  penumbra.  Umbra  is  the  Latin  word  for  shadow, 
while  penumbra  means  almost  a  shadow. 

Since  a  luminous  body  gives  out  light  in  every  direc- 
tion in  straight  lines,  it  is  said  to  radiate  light.  A 
single  line  of  light  is  called  a  ray.  A  collection  of  rays 
is  called  a  pencil.  If  the  rays  are  parallel,  it  is  a  par- 
allel pencil,  or  a  beam;  if  the  rays  diverge,  it  is  a  diver- 
gent pencil ;  if  they  converge,  a  convergent  pencil. 

194.  The  Velocity  of  Light  is  about  190,000  Miles  a 
Second.  — Light  moves  so  fast  that  it  seems  to  require  no 
time  at  all  to  pass  over  any  distance  on  the  earth.  Its 
velocity  was  first  determined  by  Roemer,  a  Danish  as- 
tronomer, in  1675,  by  observing  the  eclipses  of  Jupiter's 
moons.  Jupiter,  li^e  the  earth,  is  a  planet  which  revolves 
about  the  sun,  but  at  a  much  greater  distance  than  the 
earth.  He  is  accompanied  by  four  moons,  which  are 
eclipsed  when  they  pass  into  his  shadow ;  and  the  pre- 


LIGHT. 


'33 


cise  time  when  these  eclipses  occur  can  be  calculated  by 
astronomers.  Roemer  found  that  the  eclipses  did  not 
always  take  place  at  the  computed  time,  but  appeared 
about  sixteen  minutes  later  when  the  earth  was  farthest 
from  Jupiter  than  when  she  was  nearest  to  him.  He 
therefore  concluded  that  it  takes  light  sixteen  minutes  to 
traverse  the  difference  of  these  distances,  which  is  about 
183,000,000  miles.  Its  velocity,  then,  would  be  about 
190,000  miles  a  second,  and  this  agrees  very  nearly  with 
the  velocity  as  determined  by  wholly  different  methods. 

195.     The   Intensity   of   Light    diminishes    as    the 
Square   of  the   Distance  from    the   Luminous  Body 
increases. — In  Figure  117,  the  disk  C  D  is  held  half- 
Fig.  u7. 


way  between  the  luminous  point  L  and  the  screen  A  B. 
If  the  disk  be  held  parallel  to  the  screen,  the  diameter  of 
the  shadow  on  the  screen  will  be  twice  that  of  the  disk, 
and  its  surface  will  be  four  times  that  of  the  disk.  The 
disk  receives  all  the  light  that  the  space  covered  by  the 
shadow  would  receive  if  the  disk  were  removed.  The 
light  on  the  disk  must  then  be  four  times  as  intense  as 
that  upon  the  screen.  If  the  disk  be  held  one-third  of 
the  way  between  L  and  the  screen,  the  shadow  will  cover 
a  surface  nine  times  that  of  the  disk,  and  the  intensity  of 
the  light  on  the  disk  will  be  nine  times  as  great  as  that 
upon  the  screen ;  and  so  on.  The  intensity  of  the  light, 
then,  diminishes  as  the  square  of  the  distance  increases. 


134  LIGHT. 

196.  Reflection  and  Refraction.  —  If  a  ray  of  sun- 
light be  made  to  fall  upon  a  looking-glass  in  a  darkened 
room,  it  will  be  seen  to  be  thrown  back,  or  reflected, 
from  the  glass. 

A  piece  of  glass  cut  into  the  form  shown  in  Figure  118 
is  called  a  prism. 

Fig.  1 1 8.  Fig.  119. 


If  a  ray  of  light,  a  b,  be  allowed  to  fall  obliquely  upon 
one  side  of  such  a  prism,  as  shown  in  Figure  119,  a  part 
of  the  light  is  reflected  in  the  direction  b  c,  and  another 
part,  b  d,  enters  the  prism.  The  part  which  enters  the 
prism  is  bent  from  the  direction  of  the  original  ray. 
When  this  part  meets  the  air  at  the  opposite  side  of  the 
prism,  a  part  of  it  is  again  reflected  in  the  direction  d  e, 
and  a  part  passes  into  the  air,  taking  a  different  direc- 
tion, df,  from  that  which  it  had  while  in  the  prism. 

We  see,  then,  that  when  light  travelling  in  the  air 
meets  the  glass,  it  is  partly  reflected  and  partly  trans- 
mitted ;  and  that  when  light  travelling  in  the  glass  meets 
the  air  again,  it  is  also  partly  reflected  and  partly  trans- 
mitted. In  both  cases,  the  transmitted  portion  is  turned 
aside  from  its  course.  Light  thus  turned  aside  is  said 
to  be  refracted. 

In  general,  when  light  meets  a  transparent  medium 
different  from  that  which  it  has  been  traversing,  it  is 
partly  reflected  and  partly  transmitted.  The  trans- 
mitted portion,  when  it  enters  the  medium  obliquely,  is 
refracted. 


LIGHT.  135 

197.   The   Law  of  Reflection.  —  In  Figure  120,  we 
have  a  mirror  L  fastened  at  right  angles  to  the  rod  m  n, 

Fig.    120. 

I/ 


and  turning  upon  a  pivot  at  n.  As  the  mirror  is  turned 
to  the  right  or  left,  the  rod  passes  over  the  graduated  arc 
a  b.  If  a  ray  of  light  be  allowed  to  fall  upon  the  mirror 
in  the  direction  of  the  dotted  line  a  n,  it  will  be  reflected 
in  the  direction  of  the  line  n  b;  and  it  will  be  seen  that 
the  angle  a  n  m  is  equal  to  the  angle  b  n  m.  The  former 
is  called  the  angle  of  incidence,  and  the  latter  the  angle 
of  reflection.  If  the  mirror  be  turned,  the  direction  of 
the  reflected  ray  changes  in  such  a  way  that  the  angle  of 
incidence  always  equals  the  angle  of  reflection.  This 
is  known  as  the  law  of  reflection. 

198.  Diffused  Light.  —  Since  non-luminous  bodies 
are  not  visible  in  the  dark,  but  become  visible  when  light 
falls  upon  them,  they  must  send  to  our  eyes  some  of  the 
light  they  receive.  This  light  must  be  sent  out  in  every 
direction,  since  we  can  see  them  as  well  from  one  posi- 
tion as  another.  The  light  which  they  thus  throw  off  is 
said  to  be  diffused.  It  is  this  diffused  light  which  ena- 
bles us  to  see  the  body  itself;  while  reflected  light 
enables  us  to  see  another  body  in  it.  The  most  perfectly 
polished  mirror  does  not  reflect  all  the  light  it  receives. 
It  diffuses  a  portion,  so  that  we  see  the  mirror  as  well  as 
the  objects  reflected  in  it. 


i36 


LIGHT. 


Fig.  121. 


199.  The  Law  of  Refraction. — 
When  a  ray  of  light  passes  obliquely 
from  air  into  water,  it  is  bent  towards 
a  perpendicular  drawn  to  the  sur- 
face of  the  water.  Thus  the  ray  a  b 
(Figure  121)  is  bent  towards  the  per- 
pendicular c  d,  and  takes  the  direction 
b  e  after  passing  into  the  water.  This 
is  found  to  be  always  true  when  the 
light  passes  from  a  rarer  to  a  denser 
medium.  When  it  passes  from  a 
denser  to  a  rarer  medium,  it  is  bent  away  from  a  per- 
pendicular drawn  to  the  surface  of  the  latter  medium. 
Fig.  122.  &  is  owing  to  refraction  that  a  stick 

placed  obliquely  in  the  water  appears 
bent,  as  in  Figure  122.  Each  part  of 
the  stick  in  the  water  appears  to  be 
lifted  up  a  little  by  refraction. 

200.  Total  Rejection. — When  a  ray 
of  light  passes  from  a  denser  to  a  rarer 
medium,  as  from  water  into  air,  the  angle 
of  refraction  is  greater  than  the  angle  of  incidence  (199). 
Hence  when  light  passes  through  water  from  S  to  O 
(Figure  123),  there  is  always  a  value  of  the  angle  of 
incidence  SOB  such  that  the  an-  Fig.  I23 

gle  of  refraction  A  O  R  is  a  right 
angle.  In  this  case,  the  ray  cannot 
pass  from  the  water  into  the  air. 
If  the  incident  angle  be  made 
any  larger,  the  light  is  thrown 
back  in  the  direction  of  jg,  and  is 
said  to  be  totally  reflected. 

201.  Mirage.  —  In  hot  climates, 
especially  on  the  Sahara  in  Africa,  the  ground  has  often 
the  appearance  of  a  tranquil  lake,  on  which  are  seen 


LIGHT.  137 

reflected  houses  and  trees.  This  is  caused  by  total  re- 
flection. The  layers  of  air  near  the  ground  are  more 
heated,  and  therefore  less  dense  than  those  higher  up. 
A  ray  of  light,  then,  coming  from  A  (Figure  124)  is 

Fig.  124. 


bent  round  more  and  more  as  it  passes  down  through 
the  successive  layers  until  it  reaches  the  point  0,  where 
the  angle  of  incidence  becomes  such  that  it  is  totally 
reflected,  and  reaches  the  eye  as  if  it  came  from  B. 
The  same  will  be  true  of  light  coming  from  other  parts 
of  the  tree,  so  that  the  tree  will  appear  inverted,  as  if 
reflected  in  water.  This  phenomenon  is  called  mirage, 
and  often  deludes  the  thirsty  traveller  on  the  desert 
with  the  appearance  of  water  which  vanishes  as  he 
draws  near  it. 

Another  form  of  mirage  is  often  seen  on  the  water. 
In  this  case,  the  layers  of  air  near  the  water  are  colder 
and  more  dense  than  those  above,  so  that  the  rays  of 
light  passing  upward  from  an  object  are  bent  round 
more  and  more,  until  at  last  they  are  totally  reflected 
downward  to  the  eye  of  the  observer,  who  thus  sees 
the  object  inverted  in  the  air. 


138  LIGHT. 

202.  Path  of  Rays  through  a  Medium  tvith  Parallel 
Faces.  —  When   light   passes   through   a   medium  with 
parallel  faces,  as  a  pane  of  common  window-glass,  the 
rays  leave  this  medium  at  the  same  angle  at  'which 
they  entered  it.     Since  the  ray  in  passing  into  the  air 
is  bent  away  from  the  perpendicular  just  as  much  as 
it  was  bent  towards  it  in  passing  into   the   glass,  the 
ray  leaves  the  glass  at  the  same  angle  at  which  it  en- 
tered it;  and  its  direction,  therefore,  is  unchanged. 

203.  Path  of  Rays  through  a  Prism.  —  In  Figure  125, 

Fig.  I2S.  the  ray  of  light  O  D  on  pass- 

ing into  the  prism  A  B  C  is 
bent  towards  a  perpendicular 
drawn  to  the  surface  at  D. 
On  passing  out  into  the  air 
again,  it  is  bent  away  from  a 
perpendicular  drawn  to  the 
surface  at  K.  We  see,  then, 
that  a  ray  of  light  in  passing  through  a  prism  is 
bent  twice  in  the  same  direction  j  provided  it  meets 
neither  face  at  right  angles. 

_ 

SUMMARY. 

A  luminous  body  gives  out  light  in  every  direction, 
which  passes  through  space  in  straight  lines. 

A  single  line  of  light  is  called  a  ray;  and  a  collection 
of  rays,  a  beam,  or  pencil.  (192,  193.) 

The  velocity  of  light  is  about  190,000  miles  a  second. 


The  intensity  of  light  diminishes  as  the  square  of  the 
distance  increases.  (195.) 

When  light  falls  on  a  transparent  medium  different 
from  that  in  which  it  is  moving,  it  is  partially  reflected 


LIGHT.  139 

and    partially    transmitted.      The    transmitted    portion, 
if  it  meets  the   medium   obliquely,   is  refracted.     (196, 
199.) 
The  angle  of  reflection  equals  the  angle  of  incidence. 

(197-) 

All  bodies  diffuse  light,  and  it  is  by  means  of  this 
diffused  light  that  we  see  them.  (198.) 

On  meeting  a  rarer  medium  at  a  certain  angle,  light 
is  totally  reflected.  (200.) 

Mirage,  and  other  atmospheric  phenomena  of  the 
kind,  are  caused  by  total  reflection.  (201.) 

When  a  ray  passes  through  a  medium  with  parallel 
sides,  it  comes  out  with  its  direction  unchanged.  (202.) 

On  passing  through  a  prism,  a  ray  is  usually  bent 
twice  in  the  same  direction.  (203.) 


DISPERSION,  ABSORPTION,   INTERFERENCE,  AND 
POLARIZATION. 


\ 


204.   The  Solar  Spectrum.  —  Allow  a  beam  of  sun- 
light, S  A  (Figure  126)  to  pass  through  a  small  opening 

Fig.  126. 


into  a  darkened  room,  and  fall  upon  the   prism  P.     If 
the  prism  be  placed  at  the  proper  angle,  the  beam  of 


140  LIGHT. 

light  is  not  only  bent  from  its  course,  but  is  spread 
out  so  as  to  form  a  long  band  of  light  on  the  opposite 
wall.  This  band  is  not  white,  like  ordinary  sunlight, 
but  made  up  of  the  seven  colors  of  the  rainbow,  violet, 
indigo,  blue,  green,  yellow,  orange,  and  red.  This 
colored  band  is  called  the  solar  spectrum,  and  the 
colors  are  often  called  the  prismatic  colors. 

This  spreading  out  of  a  beam  of  light  is  called 
dispersion;  and  the  power  of  any  substance  to  pro- 
duce this  effect  is  called  its  dispersive  power.  The 
dispersive  power  of  a  substance  is  not  in  proportion 
to  its  refractive  power.  Thus  the  refractive  power 
of  flint-glass  is  almost  the  same  as  that  of  crown-glass, 
but  its  dispersive  power  is  nearly  double. 

205.  Achromatic  Prism.  —  By  combining  a  flint-glass 
prism  C  D  F  (Figure  127),  with  a  crown-glass  prism 

Fig.  127.  C  JB  F,  the  dispersive  power  of 

the  latter  may  be  neutralized,  with- 
out wholly  neutralizing  its  refract- 
ive power.  The  prism  CDF,  in 
order  to  have  the  same  dispersive 
power  as  C  B  F,  need  be  only 
half  as  thick  as  the  latter;  so  that 

the  edges  B  Cand  F  D  are  still  inclined  as  though  they 

were  sides  of  the  larger  prism  A  B  F. 

Such  a  combination  of  prisms  is  called  an  achromatic 

(colorless}  prism,  since  light  passes  through  it  without 

being  separated  into  the  prismatic  colors. 

206.  The  Prismatic  Colors  are  Simple.  —  If  all  the 
colors  of  the  spectrum  except  one  be  cut  off  by  a  screen, 
and  that  one  be  made  to  fall  on  a  second  prism  (Figure 
128),  it  will  be  again  refracted,  but  will  not  be  separated 
into  different  colors.     These  colors,  then,  are  simple. 

207.  The  Prismatic  Colors  are  unequally  Refrangi- 
ble. —  The  position  of  the  colors  in  the  spectrum  shows 


LIGHT. 


that  they  are  not  equally  refracted.      The  red  is  least, 
and  the  violet  most  refracted. 


Fig.  128. 


208.  The  Composition  of  White  Light.  —  These  ex- 
periments with  the  prism  seem  to  show  that  white  light 
is  not  simple,  but  made  up  of  the  seven  prismatic 
colors. 

The  same  may  be  shown  by  mixing  these  colors  in  the 
eye.  This  can  be  done  by  painting  them  in  the  proper 
proportions  upon  a  circular  disk  (Figure  129)  and  mak- 

Fig.  129.  Fig.  130. 


ing  this  disk  whirl  rapidly,  as  shown  in  Figure  130.    The 
impression  of  each  color  remains  in  the  eye  while  the 


142  LIGHT. 

disk  turns  completely  round,  so  that  the  seven  are  blended 
into  one,  and  the  disk  appears  'white. 

If  we  mix  two  or  more  of  these  colors,  we  get  a  tint 
different  from  any  one  of  them.  Thus  red  and  yellow 
produce  orange ;  blue  and  red,  purple ;  and  so  on.  In 
fact,  all  the  varied  colors  we  see  are  formed  by  the 
mixture  of  the  prismatic  colors. 

It  is  probable  that  white  light  is  made  up  of  only  three 
simple  colors;  for  if  red,  green,  and  blue  be  mixed,  a 
color  is  obtained  which  cannot  be  distinguished  from 
white.  Moreover,  all  the  other  prismatic  colors  can  be 
formed  by  mixing  these  three. 

According  to  some  authorities,  red,  yellow,  and  blue 
are  the  three  simple  colors ;  but  it  has  been  shown  that 
no  mixture  of  these  will  produce  all  the  prismatic  colors. 

If  the  spectrum  be  divided  into  any  two  parts,  and 
the  colors  in  each  part  be  mixed,  they  will  form  what 
are  called  complementary  colors ;  that  is,  one  will  con- 
tain what  the  other  needs  to  make  white  light. 

209.  Absorption  of  Light.  —  If  light  be  made  to  pass 
through  a  piece  of  colored  glass,  and  then  to  fall  upon  a 
prism,  the  spectrum  will  be  wanting  in  certain  colors. 
If  red  glass  is  used,  the  spectrum  will  contain  little  be- 
sides red  light ;  if  blue  or  green  glass  is  used,  the  spec- 
trum will  be  rich  in  blue  or  green,  and  deficient  in  other 
colors.     A  part  of  the  light  is  retained  in  the  glass, 
and  is  said  to  be  absorbed  by  it.     All  transparent  bodies 
absorb  a  portion  of  the  light  which  falls  upon  them.     If 
they  absorb  all  colors  equally,  they  appear  colorless ;  if 
they  absorb  some  colors  more  than  others,  their  color 
will  be  complementary  to  what  they  thus  absorb  (208). 

210.  The  Color  of  Bodies.  —  Opaque  bodies,  as  well 
as  transparent  ones,  absorb  light.     Hence,  when  white 
light  is  falling  upon  non-luminous  bodies,  they  do  not  all 
appear  of  the  same  color.     They  are  really  sifting  the 


LIGHT*  143 


light  which  they  receive,  absorbing  a  part  and  reflecting, 
diffusing,  or  transmitting  the  rest.  Their  color  depends 
upon  the  light  'which  they  rejlect  or  diffuse.  Thus  a 
body  which  absorbs  all  the  prismatic  colors  except  red 
appears  red  ;  one  which  absorbs  all  except  green  appears 
green ;  and  so  on. 

Bodies  sometimes  transmit  a  color  different  from  that 
which  they  rejlect,  and  appear  of  a  different  color  ac- 
cording as  they  are  seen  by  transmitted  or  rejlected 
light.  Gold  appears  yellow  by  reflected  light,  and  green 
by  transmitted  light,  as  may  be  seen  by  holding  a  piece 
of  gold-leaf  between  the  eye  and  the  sunshine. 

211.    Two   Rays   of  Light   may  Interfere  so   as  to 
Destroy  each  other.  —  If  a  slightly  curved  piece  of  glass 
be  pressed  down  upon  a  flat  plate  of  glass  (Figure  131), 
colored  rings  are  formed  around  the  centre  (Figure  132). 
Fig.  131.  If   any   homogeneous 

§>      (that  is,  lignt  oj 
one  kind},  as  red,  be 


used,  the  rings  are  red  and  separated  by 
black  spaces.  If  violet  light  be  used,  the 
rings  are  violet  and  smaller.  As  we  pass 
from  the  violet  end  of  the  spectrum  to  the  red  end,  the 
rings  grow  larger  and  broader.  Hence,  when  white  light 
is  used,  we  get  seven  sets  of  rings,  which  somewhat  over- 
lap one  another.  This  explains  why  there  are  several 
colors  in  each  ring.  The  dark  rings  are  caused  by  the 
interference  of  the  rays  of  light  which  are  reflected  from 
the  lower  surface  of  the  upper  glass  and  the  upper  sur- 
face of  the  lower  glass.  Hence  two  rays  of  light,  like 
two  sounds  (169),  may  interfere  so  as  to  destroy  each 
other.  These  colored  rings  are  seen  in  soap-bubbles,  and 
in  all  cases  where  there  are  two  reflecting  surfaces  very 
near  each  other.  They  are  known  as  Newton's  rings^ 
since  he  was  the  first  to  study  them. 


144  LIGHT. 

212.  Diffraction  Fringes. — Let  a  beam  of  sunlight 
fall  upon  a  small  glass  lens  *  in  a  darkened  room.     The 
light  will  be  concentrated  into  a  point  a  little  way  from 
the  lens,  and  will  then  diverge  from  it  in  a  luminous 
cone,  and  may  be  received  upon  a  screen.     Place  any 
small  opaque  body  within  this  cone  of  light,  so  that  it 
may  cast  a  shadow  upon  the  screen.     This  shadow,  in- 
stead of  being  sharply  defined,  as  we  should  expect  (193), 
is  somewhat  larger  than  it  should  be,  and  is  surrounded 
by  three  colored  fringes.     If  homogeneous  light  (211) 
is  used,  instead  of  the  fringes  we  get  bright  rings  sepa- 
rated by  dark  spaces,  the  breadth  of  the  rings  varying 
with  the  color  of  the  light.     When  white  light  is  used, 
these  different  sets  of  colored  rings  blend  so  as  to  produce 
the  fringes. 

If  the  opaque  body  is  long  and  very  narrow,  as  a  hair 
or  a  very  thin  strip  of  card,  besides  the  colored  fringes 
already  described,  others  are  seen  within  the  shadow, 
parallel  to  its  length,  and  arranged  on  the  two  sides  of 
a  central  white  line. 

When  light  is  transmitted  through  a  very  narrow  slit, 
the  fringes  become  even  more  curious  and  complicated. 

These  fringes  are  called  diffraction  fringes,  and  are 
caused  by  interference. 

213.  Double  Refraction  of  Light.  —  Figure  133  rep- 
resents a  crystal  of  Iceland  spar  (crystallized  carbonate 

Fig.  i33.  of   lime).     A    crystal    of   this 

shape  is  called  a  rhomb.  It 
has  six  faces,  which  are  equal 
parallelograms.  If  now  a  ray 
of  light  be  allowed  to  fall  on 
one  face  of  this  crystal  in  a 
darkened  room,  it  will  be 

*  The  focal  length  of  the  lens  should  be  about  an  inch. 


LIGHT.  145 

doubly  refracted,  or  divided  into  two  rays.  One  of  these 
rays  conforms  to  the  law  of  ordinary  refraction  (199), 
and  is  therefore  called  the  ordinary  ray.  The  other  ray 
does  not  conform  to  this  law.  It  is  therefore  called  the 
extraordinary  ray.  Since  the  opposite  faces  of  the  crys- 
tal are  parallel  (202),  the  ordinary  and  extraordinary  rays 
emerge  parallel  to  the  incident  ray  and  to  each  other,  but 
quite  near  together.  If,  however,  the  crystal  be  cut  into 
the  form  of  a  prism,  the  ordinary  and  extraordinary  rays, 
alter  leaving  the  prism,  will  diverge,  so  that  Fig.  134. 
we  may  easily  examine  them  separately. 
Such  a  prism  may  be  rendered  sufficiently 
achromatic  by  combining  with  it  a  second 
prism  of  glass,  whose  dispersive  power  (204, 
205)  is  different  from  that  of  the  crystal. 
This  prism  is  usually  mounted  as  shown  in  Fig- 
ure 134. 

214.  The  Ordinary  and  Extraordinary  Rays  are 
loth  Polarized.  —  Let  a  beam  of  ordinary  light  fall  on  a 
double-refracting  prism,  cut  off  the  extraordinary  ray  by 
a  screen,  and  let  the  ordinary  ray  fall  on  a  second  similar 
prism.  If  this  second  prism  be  turned  round,  we  find 
a  position  in  which  the  ray  is  refracted  singly  and  ordi- 
narily, and  another  position  in  which  it  is  refracted 
singly  but  extraordinarily.  Half-way  between  these 
two  positions,  it  will  be  doubly  refracted. 

If  the  ordinary  ray  be  cut  off,  and  the  extraordinary 
ray  be  allowed  to  fall  on  the  second  prism,  it  will  be 
singly  or  doubly  refracted  when  the  prism  has  been 
turned  round  90°  from  the  position  in  which  the  ordinary 
ray  was  singly  or  doubly  refracted. 

In  this  way,  we  find  that  neither  of  the  doubly  refracted 
rays  is  the  same  on  the  right  and  the  left  as  it  is  above 
and  below ;  in  other  words,  both  rays  have  acquired 
sides.  In  this  respect,  they  differ  from  a  ray  of  ordi- 

10 


146  LIGHT. 

nary  light,  which  is  doubly  refracted  in  every  position 
of  the  prism,  and  is  therefore  the  same  on  all  sides. 

Light  which  has  thus  acquired  sides  is  said  to  be 
polarized.  The  corresponding  sides  of  the  two  rays 
are  at  right  angles  to  each  other ;  in  other  words,  the 
extraordinary  ray  is  like  the  ordinary  ray  turned 
round  through  90°. 

/  SUMMARY. 

Nkjn  passing  through  a  prism,  a  beam  of  white  light  is 
dispersed,  and  forms  a  spectrum  of  seven  colors.  Since 
different  substances  disperse  light  differently,  two  prisms 
may  be  combined  so  as  to  form  an  achromatic  prism. 
(204,  205.) 

Prismatic  colors  are  simple  and  unequally  refrangible. 
(206,  207.) 

The  blending  of  the  seven  prismatic  colors  produces 
white  light. 

It  is  probable  that  there  are  but  three  simple  colors, 
red,  green,  and  blue. 

Two  colors,  whose  mixture  will  produce  white  light, 
are  said  to  be  complementary.  (208.) 

Different  bodies  absorb  light  of  different  colors.  It 
is  the  sifting  of  the  rays  of  light  by  absorption  which 
gives  bodies  their  color.  (209,  21  a.) 

Soap-bubbles  and  other  thin  films,  when  exposed  to 
light,  exhibit  colored  rings.  These  rings  are  always 
seen  when  light  is  reflected  from  two  surfaces  sepa- 
rated by  a  very  small  interval ;  and  they  are  caused 
by  interference.  (211.) 

When  small  bodies  are  seen  in  divergent  light,  they 
appear  surrounded  by  colored  fringes,  called  diffraction 
fringes.  These  are  caused  by  interference.  (212.) 

When  a  ray  of  light  passes  through  a  crystal  of  Ice- 


LIGHT. 


land  spar,  it  is  doubly  refracted;  one  of  the  refracted 
rays  being  called  the  ordinary,  and  the  other  the  ex- 
traordinary ray.  (213.) 

Both  the  doubly  refracted  rays  have  acquired  sides, 
and  are  said  to  be  polarized.  Their  corresponding 
sides  are  at  right  angles  to  each  other.  (214.) 


, 


OPTICAL  INSTRUMENTS. 


LENSES. 


215.  Forms  of  Lenses.  —  Lenses  are  pieces  of  glass, 
r  other   transparent   substance,    bounded  on   one   or 
both  sides  by  a  curved  surface.     The  forms  of  lenses 
used  in  optical  instruments  are  shown  in  Figure  135. 


Fig.  135- 
ABC  D  £  F 


A  is  bounded  by  two  spherical  surfaces,  and  is  called  a 
double-convex  lens.  B  has  a  spherical  surface  on  one 
side,  and  a  plane  surface  on  the  other,  and  is  called 
a  plano-convex  lens.  C  has  a  convex  surface  on  one 
side,  and  a  slightly  concave  surface  on  the  other,  and 
is  called  a  meniscus,  from  a  Greek  word  meaning  a 
crescent.  D  has  two  concave  surfaces,  and  is  called 
a  double-concave  lens.  E  has  a  concave  and  a  plane 
surface,  and  is  called  a  plano-concave  lens.  F  has  a 
concave  surface  on  one  side  and  a  slightly  convex  sur- 
face on  the  other,  and  is  called  a  concavo-convex  lens. 


148  LIGHT. 

2 1 6.  Convex  Lenses  cause  Parallel  Rays  to  Converge] 
Concave  Lenses  cause  them  to  Diverge.  —  Allow  a  beam 
of  sunlight  to  fall  upon  a  double-convex  lens  in  a  dark- 
ened room.  On  leaving  the  lens,  the  rays  ivill  converge 
to  a  point,  called  the  focus  (the  Latin  word  for  fire- 
place), since  the  heat  as  well  as  the  light  is  concen- 
trated there.  This  action  of  the  lens  upon  the  light 
will  be  understood  from  Figure  136.  It  will  be  seen 

Fig.  136. 


that  the  lens  is  somewhat  like  two  prisms  placed  back 
to  back ;  and  it  will  be  remembered  that  a  ray  of  light, 
in  passing  through  a  prism  (196),  is  bent  twice  in  the 
same  direction.  The  rays  falling  upon  the  upper  part 
of  the  lens  will  be  bent  downward,  and  those  falling 
on  the  lower  part  will  be  bent  upward,  and  they  will 
all  meet  at  F.  If  a  plano-convex  lens,  or  a  meniscus, 
be  used,  the  results  will  be  similar. 

Parallel  rays  are  made  to  -meet  at  the  focus  of  a 
convex  lens.  If,  on,  the  other  hand,  the  rays  diverge 
-from  the  focus,  they  will  become  parallel  on  passing 
through  the  lens.  If  they  diverge  from  a  point  nearer 
the  lens  than  the  focus  is,  they  will  be  so  divergent 
on  entering  the  lens  that  they  will  not  be  made  parallel 
on  leaving  it,  but  merely  less  divergent.  If  they  diverge 
from  a  point  farther  off  than  the  focus,  they  will  be 
so  little  divergent  that  they  will  become  convergent  on 
leaving  the  lens. 

If,  however,  we  use  any  one  of  the   concave  lenses, 


LIGHT. 


I49 


it  will  be  found  that  the  rays  of  light,  instead  of  con- 
verging, are  made  to  diverge,  Fig  I37 
on  leaving  the  lens  ;  as  shown 
in  Figure  137. 

Since  the  convex  lenses  all 
cause  parallel  rays  to  con- 
verge, they  are  called  converg- 
ing lenses  ;  while  the  concave 
lenses  are  called  diverging  lenses,  since  they  cause 
parallel  rays  to  diverge. 

217.  Images  formed  by  Lenses.  —  Place  a  lighted 
candle  before  a  double-convex  lens  in  a  darkened  room, 
and  a  screen  behind  it.  At  a  certain  distance  from  the 
lens,  a  distinct  inverted  image  of  the  candle  will  be 
formed  upon  the  screen.  Move  the  candle  nearer  the 
lens,  and  the  image  will  become  blurred,  but  will  be- 
come distinct  again  on  moving  the  screen  farther  from 
the  lens.  If  the  candle  be  moved  away  from  the  lens, 
the  image  becomes  blurred ;  but  it  becomes  distinct 
again  when  the  screen  is  brought  nearer  the  lens.  The 
nearer  the  candle  is  to  the  lens,  the  larger  the  image 
formed. 

The  more  convex  the  lens  used,  the  nearer  the  candle 

must  be  brought  to  it,  and  the  larger  the  image. 

• 

Fig.  138. 


i  Instead  of  using  a  more  convex  lens,  we  may  add  a 
second  convex  lens,  with  the  same  effect. 


I5O  LIGHT. 

An  image  is  formed  behind  a  lens  because  the  rays 
which  diverge  from  every  point  of  an  object  are  made 
to  converge,  on  passing  through  the  lens,  so  as  to  meet 
at  corresponding  points  behind  the  lens ;  as  shown  in 
Figure  138.  The  image  is  always  included  between 
liries  drawn  from  the  extremities  of  the  object  through 
the  centre  of  the  lens. 


SUMMARY. 

There  are  two  classes  of  lenses.  One  class  causes 
parallel  rays  to  converge,  and  the  other  causes  them  to 
diverge.  (215,  216.) 

When  objects  are  placed  in  front  of  a  converging 
lens,  images  of  them  are  formed  at  its  focus  behind  it. 

The  magnitude  of  the  image  increases  with  its  dis- 
tance from  the  lens,  and  also  with  the  convexity  of  the 
lens.  (217.) 

THE  EYE. 

218.  The  Camera  Obscura. —  If  a  converging  lens  be 
placed  before  an  opening  in  the  shutter  of  a  darkened 
room,  a  small  and  beautiful  picture  of  the  landscape 
will  be  seen  upon  a  screen  placed  a  short  distance  be- 
hind the  lens.  An  arrangement  of  this  kind  is  called 
a  camera  obscura  (Latin  for  a  dark  chamber). 

Figure  139  represents  the  camera  used  by  photog- 
raphers. C  is  a  dark  chamber ;  E  is  the  screen  of  ground 
glass  upon  which  the  image  is  received;  A  is  a  tube 
containing  the  combination  of  lenses  used  to  form  the 
image.  This  camera  can  be  adjusted  to  objects  at  dif- 
ferent distances  by  changing  the  position  of  the  screen, 
or  of  the  lenses  (which  may  be  moved  by  the  screw  /?), 
or  both. 


LIGHT. 


15* 


219.   The  Eye  is  a  Camera.  —  The  eyeball  is  com- 
posed, in  the  first  place,  of  a  tough,  firm,  spherical  case, 

Fig.  139- 


Scl  (Figure   140).     The  greater   part  of  this   case   is 
white  and  opaque,  and  is  called  the  sclerotic  coat,  or 


Fig.  140. 


M.I. 


the  white  of  the  eye.     In  front  this  case  becomes  trans- 


152  LIGHT. 

parent  and  more  convex,  and  is  called  the  cornea,  Cn. 
This  case  of  the  eye  is  kept  in  shape  by  being  filled 
with  fluids  called  the  humors.  The  aqueous  humor, 
Aq,  fills  the  corneal  chamber ;  and  the  vitreous  humor, 
Vt,  the  sclerotic  chamber.  Between  these  chambers  is 
the  double-convex  crystalline  lens,  Cry,  which  is  denser, 
and  has  a  greater  refractive  power  than  either  humor. 
The  choroid  coat,  Ch,  is  of  a  dark  color  and  highly  vas- 
cular (that  is,  full  of  vessels},  and  it  lines  the  whole 
inner  chamber  of  the  eye.  At  the  front  part  of  the 
chamber,  its  inner  surface  becomes  raised  into  ridges, 
called  the  ciliary  processes,  C.  p. 

The  iris,  Ir,  is  a  curtain  with  a  round  hole  in  the 
middle  called  the  pupil.  The  iris  has  two  sets  of 
muscular  fibres,  by  the  action  of  which  the  pupil  is 
enlarged  or  contracted.  It  gives  the  color  to  the  eye ; 
and  hence  its  name,  iris  being  the  Latin  for  rainbow. 

The  optic  nerve,  Op,  enters  the  back  of  the  eye  a  lit- 
tle way  from  the  centre  towards  the  nose.  It  then  spre'ads 
out  over  the  choroid  coat,  forming  the  retina,  Rt. 

The  eyeball  is  thus  seen  to  be  a  camera  obscura. 

In  an  ordinary  camera,  a  screen,  or  diaphragm,  is 
used  to  moderate  the  light,  and  to  cut  off  all  the  rays 
except  those  which  fall  on  the  central  part  of  the 
lens.  In  the  eye,  the  iris  acts  as  a  diaphragm,  and 
has  the  advantage  of  being  self -regulating.  It  dilates 
the  pupil  and  admits  more  light  when  the  illumination 
is  too  weak ;  it  contracts  the  pupil,  and  cuts  off  a  part 
of  the  light  when  there  is  too  much  of  it. 

220.  The  Adjustment  of  the  Eye.  —  That  the  eye 
must  adjust  itself  in  order  to  see  distinctly  at  different 
distances,  may  be  shown  by  a  very  simple  experiment. 
Stick  two  stout  needles  into  a  piece  of  wood,  so  that 
one  of  them  shall  be  about  six  inches  from  the  eye, 
and  the  other  about  twelve,  very  nearly  in  the  same 


LIGHT.  153 

direction.  If  now  you  look  at  one  needle,  you  will 
see  it  distinctly  and  without  the  least  sense  of  effort; 
but  the  image  of  the  other  will  be  blurred.  Try  now 
to  make  this  blurred  image  distinct,  and  you  find  that 
you  can  do  it,  but  not  without  effort.  In  proportion  as 
one  image  becomes  distinct,  the  other  becomes  blurred, 
and  no  effort  'will  enable  you  to  see  both  distinctly  at 
the  same  time. 

When  a  lighted  taper  is  held  near,  and  a  little  to 
one  side  of  a  person's  eye,  any  one,  on  looking  into  the 
eye  from  the  proper  position,  will  see  three  images  of 
the  flame ;  one  reflected  from  the  cornea,  one  from  the 
front  surface  of  the  crystalline  lens,  and  one  from  its 
rear  surface.  Suppose,  now,  the  person's  eye  be  steadily 
fixed  on  a  distant  object,  and  then  adjusted  to  a  nearer 
one  in  the  same  direction.  The  position  of  the  eyeball, 
of  course,  remains  the  same.  It  is  also  found  that  the 
images  reflected  from  the  cornea  and  from  the  rear  „ 
surface  of  the  lens,  remain  unchanged ;  while  the  image 
reflected  from  the  front  surface  of  the  lens  changes  its 
position  and  its  size  Figt  I4I. 

in  such  a  way  as 
to  show  that  this 
surface  has  been 
brought  forward 
and  at  the  same 
time  made  more 
convex.  The  eye  then  adjusts  itself  to  different  dis- 
tances by  altering  the  convexity  of  the  crystalline  lens. 
This  change  in  the  form  of  the  lens  is  shown  in  Figure 
141.  The  half  A  shows  the  form  of  the  lens  when  the 
eye  is  adjusted  for  distant  objects ;  and  the  half  B,  when 
it  is  adjusted  for  near  objects. 

221.   The  Structure  of  the  Retina.  —  Figure  142  rep- 
resents a  portion  of  the  retina  highly  magnified,  since 


LIGHT. 

Fig.  142.  the  whole  thickness  of  this  membrane 

does  not  exceed  -^  of  an  inch.  Next 
to  the  choroid  coat  it  consists  of  a 
great  number  of  minute  rod-like  and 
conical  bodies,  e,  arranged  side  by 
side.  This  is  the  layer  of  rods  and 
cones.  The  fibres  of  the  optic  nerve 
are  all  spread  out  between  b  and  a. 
At  the  entrance  of  the  optic  nerve, 
the  nerve  fibres  predominate,  and  the 
rods  and  cones  are  wanting.  Exactly 
at  the  centre  of  the  back  of  the  eye, 
there  is  a  slight  circular  depression 
of  a  yellowish  hue,  called  the  macula 
lutea,  or  yellow  spot.  In  this  spot, 
the  cones  are  abundant  without  the 
rods  and  nerve  fibres. 

,  222.  The  Action  of  Light  on  the  Optic  Nerve.  —  The 
fibres  of  the  optic  nerve  are  in  themselves  as  blind  as 
any  other  part  of  the  body.  To  prove  this,  we  have 
only  to  close  the  left  eye  and  with  the  right  look  steadily 
at  the  cross  on  this  page,  holding  the  book  ten  or  twelve 
inches  from  the  eye.  The  black  dot  will  be  seen  quite 


plainly  as  well  as  the  cross.  Now  move  the  book 
slowly  towards  the  eye,  which  should  be  kept  fixed  on 
the  cross.  At  a  certain  distance  the  dot  will  suddenly 
disappear ;  but  on  bringing  the  book  still  nearer  it  will 
come  into  view  again.  Now  it  is  found,  that,  when 
the  dot  disappears  its  image  falls  exactly  upon  the  point 
where  the  optic  nerve  enters  the  eye,  and  whe^e  there 
are  no  rods  and  cones,  but  merely  nerve  fibres.  Again, 
the  yellow  spot  is  the  most  sensitive  part  of  the  retina, 
though  it  contains  no  nerve  fibres. 


LIGHT.  155 

It  would  appear,  then,  that  the  optic  nerve  is  not 
directly  affected  by  light,  but  only  through  the  rods 
and  cones.  These  remarkable  bodies  are  like  so  many 
finger-points,  endowed^  with  a  touch  delicate  enough  to 
feel  the  impulses  of  light  and  communicate  the  impres- 
sion to  the  optic  nerve. 

223.  The  Sensation  of  Light  may  be  excited  by  Other 
Causes.  —  The    sensation   of   light   may   be   excited   by 
any  thing  which  can  excite  the  optic  nerve.     Thus  an 
electric  shock  sent  through  the  eye  causes  an  apparent 
flash  of  light.     If  the  finger  be  pressed  on  one  side  of 
the   eyeball,    a   luminous   image  is  seen.      In  the   same 
way,  a  blow  on  the  head  may  make  one  "  see  stars." 

224.  The  Duration  of  the  Impression  on  the  Retina. 
—  The  impression  made  by  light  on  the  retina  does  not 
cease  the  instant  the  light  is  removed,  but  lasts  about  the 
eighth  of  a  second.     If  the  impressions  are  separated  by 
a  less  interval,  they  appear  continuous.     Thus,  if  a  stick 
with  a  spark  of  fire  at  the  end  be  whirled  round  rapidly, 
it  gives  the  impression  of  a  circle  of  light.     The  spokes 
of  a  wheel  in  rapid  motion  cannot  be  distinguished. 

The  optical  toy  called  the  thaumatrope  illustrates  the 
same  principle.  One  form  of  it,  known  as  the  zoetrope, 
consists  (Figure  143)  of  a  cylindrical  FJ 

paper  box  turning  on  an  upright  axis. 
Near  the  top  of  the  box  is  a  row  of 
upright  slits.  The  successive  positions 
which  a  moving  body  assumes  are  rep- 
resented in  order  upon  a  strip  of  pa- 
per ;  and  this  paper  is  put  within  the 
box,  which  is  then  whirled  round  rap- 
idly. If  we  look  at  the  pictures  through  the  slits, 
they  come  before  the  eye  one  after  another,  and  the 
impression  of  each  picture  lasts  till  the  next  ar- 
rives, so  that  they  all  blend  into  one,  and  the  object 


IS*  LIGHT. 

appears  to  be  really  going  through  the  evolutions  repre- 
sented. 

225.  Irradiation.  —  A  white  or  very  bright  object 
seen  against  a  black  ground  appears  larger  than  it 
really  is;  while  a  black  object  on  a  white  ground  ap- 
pears smaller  than  it  really  is.  The  two  circles  given 
in  Figure  144  illustrate  this.  The  black  one  and  the 

Fig.  144. 


white  one  are  of  just  the  same  size,  but  the  former 
appears  to  be  the  smaller. 

This  effect  is  called  irradiation.  It  arises  from  the 
fact  that  the  impression  produced  by  a  bright  object  on 
the  retina  extends  beyond  the  outline  of  the  image. 

We  have  a  marked  case  of  irradiation  in  the  new 
moon,  which  seems  much  larger  than  the  old  one  which 
it  is  said  to  "  hold  in  its  arms." 

226.  The  Sensibility  of  the  Retina  is  easily  ex- 
hausted.—  When  we  look  at  a  bright  light,  and  then 
turn  the  eye  towards  a  moderately  lighted  surface,  a  dark 
spot  is  seen ;  showing  that  the  part  of  the  retina  on 
which  the  bright  light  fell  has  lost  for  the  moment  its 
sensibility,  or  become  blind.  If  the  bright  object  be  of 
one  color,  the  part  of  the  retina  on  which  its  image  falls 
becomes  insensible  to  rays  of  that  color,  but  not  to  those 
of  other  colors.  If  a  red  wafer  be  stuck  upon  a  sheet  of 
white  paper,  and  viewed  steadily  for  some  time  with  one 
eye,  and  then  the  eye  be  turned  to  another  part  of  the 


LIGHT.  157 

paper,  a  greenish  spot  will  appear  of  the  size  and  shape 
of  the  wafer.  The  red  image  has  made  the  retina  blind 
to  red  light,  but  it  has  left  it  sensitive  to  the  remaining 
colors  which  make  up  white  light  (208)  ;  and  when  red 
is  taken  from  white  light  the  combination  of  the  other 
colors  gives  a  greenish  hue.  If  the  wafer  is  green,  the 
spot  seen  will  be  red. 

227.  Color- Blindness. — In  some  persons  the  retina 
appears  to  be  affected  in  one  and  the  same   way  by 
different  colors,  or  even  by  all  colors.     The  most  com- 
mon form  of  this  color-blindness,  as  it  is  called,  is  an 
inability  to  distinguish  red  and  green.     Thus  to  many 
persons  the  leaves  of  the  cherry-tree  and  its  fruit  seem 
of  the  same  color.     In  some  cases,  persons  who  were 
color-blind  without  being   aware  of  it,   and  who    have 
been  employed   on  railways,  have    mistaken   the   color 
of   signal-lights,   and    serious    accidents   have   been   the 
result. 

This  blindness  may  arise  either  from  a  defect  in  the 
retina,  or  from  some  peculiarity  in  the  absorptive 
powers  of  the  humors  of  the  eye. 

228.  The  Optical  Axis  and  the  Visual  Angle.  —  A 
line  drawn  from  the  centre  of  the  yellow  spot  through 
the  centre  of  the  pupil  is  called  the  optical  axis.     When 
we  look  at  any  object  we  must  turn  the  eye  so  as  to 
direct  this  axis  toward  it.     This  gives  us  the  direction 
of  the  object. 

The  image  of  an  object  on  the  retina  is  contained 
between  lines  drawn  from  the  extremities  of  the  object 
through  the  centre  of  the  crystalline  lens  (217).  The 
angle  contained  between  lines  thus  drawn  is  called  the 
visual  angle  of  the  object,  and  of  course  measures  the 
length  of  the  image  on  the  retina.  All  objects  which 
have  the  same  visual  angle  form  images  of  the  same 
length  on  the  retina. 


158  LIGHT. 

229.  How  we  estimate  the  Size  of  a  Body.  —  The 
visual  angle  evidently  gives  us  no  information  as  to  the 
real  size  of  a  body ;  for  we  see  from  Figure  145  that  the 

Fig-  145- 


visual  angle  of  a  body  diminishes  as  its  distance  in- 
creases, and  also  that  bodies  at  different  distances  may 
have  the  same  visual  angle,  though  they  are  not  of  the 
same  size.  Thus  A  B  and  Af  B1  are  the  same  object, 
but  /  Ar  B1  which  is  farther  off  has  the  smaller  visual 
angle.  Again  C  D  and  A1  B1  have  the  same  visual 
angle,  but  A1  B1  is  the  larger. 

Hence,  we  must  know  the  distance  of  a  body  in  order 
to  estimate  its  size;  but  when  we  know  this  distance 
we  estimate  its  size  instinctively.  Thus  a  chair  at  the 
other  side  of  the  room  has  a  visual  angle  only  half  as 
large  as  that  of  a  chair  half  as  far  from  the  isye,  yet  we 
cannot  make  it  seem  smaller  if  we  try.  If  we  are  in 
any  way  deceived  as  to  the  distance  of  an  object,  we 
are  also  deceived  as  to  its  size. 

230.  How  we  estimate  the  Distance  of  an  Object.  — 
If  we  refer  to  Figure  146,  we  see  that  when  the  eyes  are 
directed  to  a  distant  object,  as  C,  they  are  turned  inward 
but  slightly ;  while  they  are  turned  inward  considerably 
when  directed  to  the  nearer  object  D.  The  muscular 
effort  we  have  to  make  in  turning  the  eyes  inward  so  as 
to  direct  them  upon  an  object  is  one  of  the  best  methods 
we  have  of  estimating  its  distance. 

We  also  judge  of  the  distance  of  an  object  from  the 
distinctness  with  which  we  see  it.  The  more  obscure 


LIGHT.  159 

it  is,  the  more  distant  it  seems.     It  is  for  this  reason  that 
objects  seen  in  a  fog  sometimes  appear  enormously  large. 

Fig.  146. 


They  appear  indistinct,  and  we  cannot  rid  ourselves  of 
the  impression  that  they  are  far  off;  and  hence  they  seem 
large,  though  they  may  really  be  small  and  near  us. 

When  we  know  the  real  size  of  an  object  we  judge  of 
its  distance  from  the  visual  angle  ;  but  we  judge  of  the 
distance  of  unknown  objects  mainly  by  comparing  it 
with  the  distance  of  known  objects.  This  is  one  reason 
why  the  moon  appears  larger  near  the  horizon  than  over- 
head, though  she  is  really  nearer  in  the  latter  case. 
When  she  is  on  the  horizon  we  see  that  she  is  beyond 
all  the  objects  on  the  earth  in  that  direction,  and  there- 
fore she  seems  farther  off  than  when  overhead,  where 
there  are  no  intervening  objects  to  help  us  to  judge  of 
the  distance. 

231.  Why  Bodies  near  us  appear  Solid.  — Hold  any 
solid  object,  as  a  book,  about  a  foot  from  the  eyes,  and 
look  at  it  first  with  one  eye  and  then  with  the  other.  It 
will  be  seen  that  the  two  images  of  the  object  are  not 
exactly  alike.  With- the  right  eye  we  can  see  a  little 
more  of  the  right  side  of  the  object,  and  with  the  left 
eye  a  little  more  of  its  left  side.  It  seems  to  be  the 
blending  of  these  two  pictures  which  causes  objects  to 
appear  solid. 


1 60  LIGHT. 

232.  The  Stereoscope.  —  The  principle  just  stated  ex- 
plains the  action  of  the  stereoscope.     Two  photographs 

Fig.  147.  °f    an    object    are     taken    from 

slightly  different  points  of  view, 
so  as  to  obtain  pictures  like  those 
formed  in  the  two  eyes.  These 
photographs  are  placed  before 
the  eyes  in  such  a  manner  that 
each  eye  sees  only  one,  but  both 
are  seen  in  the  same  position. 
The  pictures  (Figure  147)  are 
placed  at  A  and  JB.  The  rays 
of  light  from  them  fall  upon  the 
lenses  m  and  n,  and  in  passing 
through  them  are  bent  so  that 
they  enter  the  eye  as  if  they 
came  from  the  direction  C.  The 
lenses  are  portions  of  a  double- 
convex  lens,  arranged  as  shown  in  the  figure. 

233.  The  Laws  of  Distinct  Vision.  —  To  see  an  ob- 
ject distinctly,  a  clear  image  of  it  must  be  formed  on 
the  retina.     When  an  object  is  brought  quite  near  the 
eye,  it  becomes  indistinct ;  showing  that  there  is  a  limit 
to  the  power  which  the  eye  has  (220)  of  adjusting  itself 
for  different  distances.     The  rays  are  now  so  divergent 
that  the  lens  cannot  bring  them  to  a  focus  on  the  retina. 
The  nearest  point  at  ivJiich  a  distinct  image  is  formed 
upon  the  retina  is  called  the  near  point  of  vision,  and 
the  greatest  distance  at  which  such  an  image  is  formed 
is  called  the  far  point.     In  perfectly  formed  eyes,  the 
near  point  is  about  3^  inches  from  the  eye,  and  the  far 
point  is  infinitely  distant.     In  such  eyes,  parallel  rays  are 
brought  to  a  focus  exactly  at  the  retina  when  the  eye  is 
at  rest ;  that  is,  when  the  crystalline  lens  is  of  its  natural 
convexity.    The  pupil  of  the  eye  is  so  small  that  the  rays 


LIGHT.  l6l 

which  fall  upon  it  from  objects  1 8  or  20  inches  distant 
diverge  so  little  that  they  may  be  regarded  as  parallel. 
The  distance  of  the  near  and  far  ^0/7z£y,_however,  is 
not  the  same  for  all  eyes.  In  some  cases,  the  near  point 
is  considerably  less  than  3^  inches  from  the  eye,  while 
the  far  point  is  only  8  or  10  inches.  In  other  cases,  the 
near  point  is  12  inches  from  the  eye,  and  the  far  point 
infinitely  distant.  The  former  are  called  near-sighted 
eyes  ;  the  latter,  far-sighted  ones. 

It  was  once  thought  that  near-sightedness  was  due  to 
the  too  great  convexity  of  the  cornea  or  the  crystalline 
lens,  or  of  both,  and  far-sightedness  to  the  too  slight 
convexity  of  the  same.  But  actual  measurement  has 
shown  that  their  real  cause  lies  in  the  shape  of  the 
eyeball,  which  in  far-sighted  people  is  flattened,  and 
in  near-sighted  people  elongated,  in  the  direction  of 
the  axis.  In  Figure  148,  the  curve  N  shows  the  form 

Fig.  148. 


of  the  normal,  or  perfect  eye  ;  N',  of  the  far-sighted  eye  ; 
and  N",  of  the  near-sighted  eye.  The  eye  is  represented 
as  at  rest,  and  we  see  that  the  parallel  rays  A  and  A 
are  brought  to  a  focus  on  the  retina  of  the  normal  eye, 
while  only  the  convergent  rays  A  and  A  are  brought  to 
a  focus  on  the  retina  of  the  far-sighted  eye,  and  only  the 
divergent  rays  A"  on  the  retina  of  the  near-sighted  eye. 
A"  then  is  the  far  point  for  the  near-sighted  eye,  since 
the  lens  has  now  its  least  convexity ;  and  this  point  must 

ii 


1 62  LIGHT. 

be  within  18  or  20  inches,  since  the  rays  from  an  object 
farther  off  are  virtually  parallel  and  cannot  be  brought 
to  a  focus  on  the  retina.  The  near  point  must  be  less 
than  for  the  normal  eye,  since  the  retina  is  farther  from 
the  lens,  and  therefore  rays  of  greater  divergence  can  be 
brought  to  a  focus  upon  it.  In  the  far-sighted  eye,  the 
retina  is  nearer  the  lens  than  in  the  normal  eye ;  hence 
the  near  point  is  farther  away.  While,  then,  the  normal 
eye  sees  distant  objects  distinctly  without  adjustment,  the 
far-sighted  eye  must  adjust  itself  to  see  them. 

The  defect  of  far-sighted  eyes  can  be  in  great  meas- 
ure remedied  by  wearing  convex  glasses,  'which  help  to 
bring  the  rays  to  a  focus  on  the  retina,  and  thus 
diminish  the  distance  of  the  near  point.  The  defect 
of  near-sighted  eyes  can  be  remedied  by  the  use  of  con- 
cave glasses,  which  render  parallel  rays  divergent, 
and  thus  increase  the  distance  of  the  far  point. 

T\\e  first  law  of  distinct  vision,  then,  is  that  a  distinct 
image  of  the  object  must  be  formed  on  the  retina. 

Again,  it  is  well  known  that,  as  evening  approaches, 
objects  become  indistinct.  Here,  of  course,  the  image 
on  the  retina  is  distinct,  but  it  is  not  brilliant  enough 
to  produce  the  proper  effect  upon  the  optic  nerve. 

The  second  law  of  distinct  vision,  then,  is  that  the 
image  must  be  sufficiently  illuminated. 

Again,  some  objects  are  so  small  that  they  cannot  be 
seen,  however  much  they  may  be  illumined.  Here  the 
image  is  too  minute  to  affect  the  optic  nerve. 

The  third  law  of  distinct  vision,  then,  is  that  the 
image  must  be  of  sufficient  magnitude. 

234.  Old  Eyes.  —  As  the  eye  grows  old  it  loses  its 
power  of  adjustment,  the  crystalline  lens  becoming  less 
elastic.  Hence  old  eyes  can  see  distinctly  only  distant 
objects.  This,  however,  is  quite  a  different  thing  from 
far-sightedness.  In  the  far-sighted  eye,  there  is  no  lack 


LIGHT.  163 

of  power  to  change  the  convexity  of  the  lens,  but  this 
power  becomes  useless  because  of  the  distance  of  the 
retina. 

This  defect  of  vision  caused  by  age  can  be  remedied 
by  the  use  of  convex  glasses. 


SUMMARY. 

The  camera  obscura  is  an  apparatus  by  which  an 
image  of  an  object  can  be  formed  on  a  screen  in  af  dark- 
ened chamber.  (218.) 

The  eye  is  a  camera  obscura.     (219.) 

Ths  eye  adjusts  itself  to  light  of  varying  intensity  by 
varying  the  size  of  the  pupil.  (219.) 

It  adjusts  itself  to  various  distances  by  changing  the 
convexity  of  the  crystalline  lens.  (220.) 

The  optic  nerve  is  blind. 

The  light  acts  upon  the  rods  and  cones,  which  trans- 
mit the  impression  to  the  optic  nerve.  (222.) 

Any  thing  which  excites  the  optic  nerve  produces  the 
sensation  of  light.  (223.) 

The  impression  on  the  retina  lasts  a  short  time  after 
the  object  which  produced  it  has  been  removed.  (224.) 

The  impression  of  a  bright  object  extends  beyond  the 
image,  giving  rise  to  irradiation.  (225.) 

The  sensitiveness  of  the  retina  for  any  color  is  readily 
exhausted.  (226.) 

We  judge  of  the  direction  of  an  object  by  the  direc- 
tion of  the  axis  of  the  eye  when  turned  towards  it. 

The  visual  angle  of  an  object  depends  on  its  size  and 
distance.  (228.) 

We  judge  of  the  size  and  distance  of  an  object  by 
means  of  its  visual  angle,  the  direction  of  the  optical 
axis,  and  the  distinctness  of  the  image.  (229,  230.) 


164  LIGHT. 

Near  bodies  seem  solid,  because  the  images  in  the 
two  eyes  are  not  exactly  alike.  (231.) 

The  stereoscope  causes  pictures  on  a  plane  surface  to 
appear  solid.  (232.) 

In  order  that  vision  may  be  distinct,  a  distinct  image 
must  be  formed  on  the  retina,  the  image  must  be  suffi- 
ciently illuminated,  and  must  have  sufficient  magnitude. 

Perfect  eyes  can  adjust  themselves  to  any  distance  from 
3^  inches  to  infinity.  Near-sighted  eyes  can  adjust  them- 
selves only  to  short  distances,  and  far-sighted  eyes  only 
to  long  distances.  (233.) 

Eyes  lose  their  power  of  adjustment  as  they  grow  old. 

(234-) 

Near-sightedness  and  far-sightedness  are  due  to  defec- 
tive forms  of  the  eyeball.  These  defects  and  that  caused 
by  age  can  be  partially  remedied  by  the  use  of  glasses. 
(233>  234.) 

THE  MICROSCOPE  AND  THE  TELESCOPE. 

235.  The  Simple  Microscope.  —  We  have  seen  (233) 
that  an  object  must  form  upon  the  retina  an  image  of  a 
certain  magnitude,  in  order  to  be  distinctly  seen.  Now 
the  magnitude  of  the  image  may  be  increased  indefi- 
nitely by  bringing  the  object  nearer  the  eye ;  but  when 
it  is  brought  too  near,  the  eye  is  not  able  to  bring  the 
rays  from  it  to  a  focus  on  the  retina.  We  may  accom- 
plish this,  however,  by  the  aid  of  a  convex  lens.  Such  a 
lens  is  the  simplest  form  of  a  microscope.  It  is  called 
a  microscope  (from  two  Greek  words  meaning  to  see 
small  things)  because  it  enables  us  to  see  things 
smaller  than  the  unaided  eye  can  distinguish.  The 
more  convex  the  lens,  the  nearer  can  the  object  be 
brought  to  the  eye,  and  the  larger  will  be  the  image 
on  the  retina. 


LIGHT.  165 

236.  The  Compound  Microscope. — In  Figure  149,  we 
have  what  is  called  a  compound  microscope.      M  is  a 


Fig.  149- 


lens;  A  B  is  an  object  placed  near  it.  An  enlarged 
image  of  A  B  is  formed  at  a  b,  and  this  image  is  viewed 
through  the  lens  N,  in  the  same  way  that  an  object  is 
viewed  with  the  single  lens  of  a  simple  microscope. 

The  lens  Mis  called  the  object-glass  or  the  objective; 
and  TV7",  the  eye-piece.  The  latter  is  usually  a  combina- 
tion of  two  lenses. 

We  have  seen  (216)  that  a  convex  lens  causes  the  rays 
passing  through  it  to  meet  at  a  focus.  In  reality,  how- 
ever, this  focus  is  not  exactly  the  same  for  all  the  rays. 
Those  falling  near  the  margin  of  the  lens  meet  a  little 
sooner  than  those  falling  upon  its  centre,  causing  what 
is  called  aberration.  The  more  convex  the  lens,  the 
greater  the  aberration,  and  the  less  distinct  the  image. 
This  aberration  can  be  diminished  by  diminishing  the 
size  of  the  lens,  so  that  all  the  rays  must  fall  near  its 
centre.  Hence  the  objective  of  a  compound  microscope, 
which  is  a  very  convergent  lens,  is  made  very  small. 

The  magnifying  power  of  a  microscope  is  commonly 
expressed  in  diameters.  If  it  makes  the  breadth  of  the 
object  appear  50  times  as  great  as  it  really  is,  it  is  said 
to  magnify  50  diameters.  Of  course  the  surface  of  the 
object  is  increased  as  the  square  of  its  diameter;  or  in 
this  case  2,500  times.  The  most  powerful  compound 
microscopes  magnify  1,500  diameters,  or  even  more. 

Of  course,  there  is  no    more   light   on   this   enlarged 


1 66  LIGHT. 

image  than  there  is  on  the  object  itself;  hence  the  ob- 
ject must  be  strongly  illuminated  in  order  that  the  light 
when  thus  diluted  may  be  sufficient  to  affect  the  eye. 

2.37.  The  Telescope.  —  As  an  object  is  moved  farther 
and  farther  from  the  eye,  its  image  becomes  smaller  and 
smaller,  until  at  last  it  may  cease  to  affect  the  eye,  even 
though  the  object  itself  may  be  very  large.  An  instru- 
ment for  examining  distant  objects  is  called  a  telescope.^ 
The  word  is  made  up  of  two  Greek  words  meaning  to 
see  far  off.  Its  construction  (Figure  150)  is  very  much 

Fig.  150. 


like  that  of  the  compound  microscope.  It  has  an  object- 
glass^  M,  for  forming  an  image,  a  b,  of  the  object  A  B, 
and  an  eye-piece,  N,  for  examining  this  image.  It  differs 
from  the  microscope  mainly  in  the  fact  that  the  image 
is  always  smaller  than  the  object.  Since  the  object  is 
very  distant,  the  rays  which  fall  upon  the  object-glass 
are  virtually  parallel ;  hence  this  glass  may  have  a  great 
diameter  without  making  the  image  indistinct  through 
aberration.  The  larger  the  diameter  the  better,  since  it 
will  collect  and  concentrate  the  more  light  on  the  image. 

The  size  of  the  image  increases  with  its  distance 
from  the  object-glass.  To  make  this  distance  as  great 
as  possible,  the  object-glass  has  very  slight  convexity. 

The  object-glass  of  the  telescope  is  made  as  large  as 
possible,  with  very  slight  convexity ;  while  that  of  the 
microscope  is  made  as  small  as  possible,  with  very 
great  convexity.  The  eye-piece  is  the  same  in  both 


LIGHT. 


i67 


instruments.     The   magnifying  is  chiefly  done  by  th& 
eye-piece. 

>We  have   seen  that  light  is  dispersed  when  passing 
through  a  prism  (204)  ;   and  that  a  double-convex  lens 
is  somewhat  like   two  prisms  placed  back  to     Fig.  151. 
back   (215).     Such  a  lens   therefore   disperses 
the  light  which  passes  through  it,  giving  rise 
to  colored  fringes  round  the*  image.     This  can 
be  prevented  by  the  use  of  a  second  lens  made 
of  glass   of  different   dispersive  power   (204). 
A  lens   thus   corrected  (Figure  151)   is  called 
an  achromatic  (colorless)  lens. 

238.  The  Terrestrial  Telescope.  —  The  image  in  the 
telescope  described  above  will  be  inverted.  An  erect 
image  may  be  obtained  by  using  additional  lenses. 
The  first  inverted  image  (Figure  152)  is  formed  at  a  6. 

Fig.  152. 


The  lens  P  renders  the  rays  diverging  from  this  image 
parallel,  and  J§  brings  them  to  a  focus  again  at  a1  b1. 
These  two  lenses  then  act  as  one,  and  form  an  inverted 
image  of  the  inverted  image,  .or  an  erect  image. 

Fig.  153- 


239.  The  Opera -Glass.  —  M (Figure  153)  is  the  ob- 
ject-glass, and  is  a  converging  lens.  R  is  the  eye-piece, 
and  is  a  diverging  lens.  The  rays  of  light  coming  from 


i68 


LIGHT. 


the  ends  A  and  B  of  the  object  would  be  brought  to  a 
focus  at  a  b,  where  an  inverted  image  would  be  formed. 
But  on  falling  upon  the  eye-piece  R  they  are  turned 
aside,  so  that  they  enter  the  eye  as  if  they  came  from 
the  points  a'  and  b1.  Hence  the  eye  sees  the  object  A  B 
erect  and  under  a  greater  visual  angle  (228)  than  if  it 
had  been  viewed  directly. 

The  telescope  invented  by  Galileo  was  an  opera-glass. 

,THE  MAGIC  LANTERN. 

240.  In  the  photographic  camera  (Figure  139)  a  small 
inverted  image  of  the  object  is  formed  upon  the  glass 
screen  E.  If  E  be  a  transparent  picture,  and  'a  strong 
light  be  sent  through  it  from  behind,  an  enlarged  and 
upright  image  of  the  picture  will  be  formed  by  the  lenses 
in  the  tube  A,  and  may  be  received  upon  a  screen  in  a 
darkened  room.  The  nearer  the  picture  is  to  the  lens, 
the  farther  off  and  the  larger  will  be  the  image. 

An  instrument  for  thus  projecting  pictures  upon  a 
screen  is  called  a  magic  lantern. 


SUMMARY. 

The  microscope  is  an  .instrument  which  enables  the 
eye  to  see  an  object  at  less  distance  than  it  otherwise 
could.  With  the  simple  microscope,  the  object  is  viewed 
directly ;  with  the  compound  microscope,  an  enlarged 
image  of  the  object  is  viewed.  (235,  236.) 

The  telescope  is  an  instrument  for  viewing  a  distant 
object.  An  image  of  the  object  is  formed  in  the  focus 
of  the  object-glass  and  viewed  with  a  microscope. 

The  object-glass  of  a  telescope  is  made  achromatic  by 
combining  lenses  of  different  dispersive  power.  (237.) 


LIGHT.  169 

In  terrestrial  telescopes,  two  or  more  lenses  are  com- 
bined so  as  to  make  the  object  appear  upright.  (238.) 

The  magic  lantern  forms  a  magnified  image  of  an 
object  upon  a  screen  in  a  darkened  room.  (240.) 

MIRRORS. 

241.  Plane  Mirrors.  —  A  mirror  is  a  smooth  reflect- 
ing surface.     If  the  surface  \sjlat,  it  is  a  plane  mirror. 

In  Figure  154,  suppose  a  point  of  light  A  to  be  in 
front  of  the  plane  mirror  N  M.  The  rays  diverging 
from  A,  as  A  I?  and  A  C,  are  re-  Fig.  154- 

fleeted  from  the  mirror  so  as  to 
make  the  angle  of  reflection  equal 
to  that  of  incidence.  After  reflec- 
tion they  enter  the  eye  O  just  as  if 
they  came  from  the  point  a.  This 
point  will  therefore  appear  to  be 
just  as  far  behind  the  mirror  as 
A  is  in  front  of  it. 

A  plane  mirror  simply  alters  the  direction  of  the 
rays,  and  makes  them  appear  to  come  from  a  point  as 
far  behind  the  mirror  as  the  object  is  in  front  of  it.  Now, 
as  we  always  see  an  object  in  the  direction  which  the 
rays  have  on  entering  the  eye,  the  object  will  appear  to 
be  behind  the  mirror.  No  image  is  formed;  for,  in 
order  to  form  an  image,  the  rays  diverging  from  the 
object  must  be  made  to  converge  so  as  to  meet. 

242.  Concave  Mirrors. — A  concave  mirror  is  a  por- 
tion of  the  inner  surface  of  a  hollow  sphere. 

In  Figure  155,  C  is  the  centre  of  the  sphere  of  which 
the  mirror  is  a  part.  The  radii  C  A,  C  B,  and  C  D 
are  perpendicular  to  the  surface  of  the  mirror  at  the 
points  A,  JB,  and  D.  Parallel  rays,  as  H,  G,  and  L,  on 
meeting  the  mirror,  are  reflected  so  as  to  make  the  angle 


I7O  LIGHT. 

of  reflection  equal  to  that  of  incidence ;  that  is,  making 
CB  H  equal  to  CB  F,  C  D  G  to  CDF,  etc.     Hence 

Fig-  iSS- 


the  reflected  rays  are  made  to  converge.  If  the  mirror 
is  not  more  than  8°  or  10°  in  breadth,  the  rays  will  all 
meet  at  F,  half-way  between  C  and  A..  This  point  is 
called  the  principal  foctis  of  the  mirror. 

If  rays  diverge  from  a  point  nearer  the  mirror  than 
the  principal  focus  is,  they  will  still  diverge  on  reflec- 
tion. If  they  diverge  from  a  point  farther  off  than 
the  principal  foc2ts,  they  'will  converge  on  reflection. 

243.  Images  formed  by  Concave  Mirrors.  —  If  an 
object  be  in  front  of  a  concave  mirror,  and  outside  of  the 
principal  focus,  an  inverted  image  of  it  will  be  formed 

Fig.  156. 


in  front  of  the  mirror  (Figure  156)  ;  for  the  rays  which 
diverge  from  it  will  be  made  to  converge  and  meet. 
The  size  of  the  image  will  increase  with  its  distance 
from  the  mirror ;  and  its  distance  from  the  mirror 
becomes  greater  as  the  mirror  is  made  less  concave, 
and  as  the  object  is  brought  nearer. 


LIGHT.  171 

244.  Convex  Mirrors.  —  A  convex  mirror  is  a  portion 
of  the  surface  of  a  sphere. 

Such  a  mirror  (Figure  157)  renders  parallel  rays  di- 
vergent, and  divergent  rays  more  divergent.  Hence 

Fig-  157- 


an  object  reflected  in  it  appears  smaller  than  it  really  is. 

A  concave  mirror  affects  the  rays  like  a  convex  lens, 
and  a  convex  mirror  like  a  concave  lens. 

245.  The  Reflecting  Telescope.  —  A  concave  mirror 
may  be  used  instead  of  the  object-lens  of  a  telescope,  as 

Fig.  158. 


is  shown  in  Figure  158.  The  rays  from  an  object  falling 
upon  the  concave  mirror  M  are  reflected  so  as  to  form 
an  image  at  the  focus,  and  this  image  is  viewed  with  the 
eye-piece  o.  As  the  image  is  formed  by  reflected  light, 
the  instrument  is  called  a  reflecting  telescope.  The  or- 
dinary telescope  is  called  a  refracting  telescope,  since 
the  image  is  formed  by  refracted  light. 


172  LIGHT. 

246.  Parabolic  Mirrors.  —  The  mirror  shown  in  Fig- 
ure 159  has  what  is  called  a  parabolic  surface,  and  is 
Fig  I5g  therefore    called    a  para- 

bolic mirror.  The  point 
F  is  called  the  focus,  and 
the  line  A  X  the  axis,  of 
the  mirror.  If  parallel 
rays  fall  upon  such  a 
mirror,  they  are  reflected 
exactly  to  the  focus  F, 
whatever  may  be  the 
breadth  of  the  mirror. 
On  the  other  hand,  if  a  light  be  placed  at  the  focus, 
its  rays  will  be  reflected  from  the  mirror  in  parallel 
lines.  This  is  because  a  parabolic  surface  is  curved  in 
such  a  way  that  if  a  perpendicular  be  drawn  to  any 
point,  as  M,  the  angle  which  it  makes  with  the  line 
ML,  drawn  parallel  to  the  axis,  is  equal  to  the  angle 
it  makes  with  the  line  M  F  drawn  to  the  focus. 

Parabolic  mirrors  are  used  for  the  lanterns  placed  in 
front  of  locomotive  engines  and  in  many  light-houses. 


SUMMARY. 

An  object  is  seen  reflected  in  a  plane  mirror  without 
enlargement,  but  it  appears  as  far  behind  thje  mirror  as  it 
really  is  before  it.  (241.) 

An  inverted  image  of  ah  object  placed  outside  the 
principal  focus  is  formed  in  front  of  a  concave  mirror. 

A  concave  mirror  may  be  used  in  place  of  the  object- 
glass  in  a  telescope.  (243,  245.) 

In  a  convex  mirror  an  object  appears  smaller  than  it 
really  is.  (244.) 

A  parabolic  mirror  renders  the  rays  which  diverge 
from  its  focus  parallel.  (246.) 


HEAT. 


PROPAGATION  OF  HEAT. 

247.  Heat  is  Radiated  in  all  Directions.  —  When 
we  come  near  a  stove  we  feel  its  heat,  no  matter  on  what 
side  of  it  we  may  be  ;  that  is,  the  stove  radiates  its  heat 
in  all  directions. 

Radiant  heat,  like  light,  diminishes  in  intensity  as 
the  square  of  the  distance  increases,  and  for  the  same 
reason. 

248.  Heat  traverses  Space  in   Straight  Lines  and 
with  the  Velocity  of  Light.  —  Heat  and  light  come  to 
the  earth  together  in  the  sun's  rays,  and  we  have  seen 
that  these  move  in  straight  lines  and  with  a  velocity  of 
about  190,000  miles  a  second. 

249.  Luminous  and  Obscure  Heat.  —  Heat  which  is 
radiated  from  a  non-luminous  source,  as  from   a  ball 
heated  below  redness,  is  called  obscure  heat ;  while  that 
radiated  from  a  luminous  source,  as  from  the  sun  or 
from  a  ball  heated  to  redness,  is  called  luminous  heat. 

250.  Diathermanous  Bodies.  —  Some  substances,  as 
air,  allow  radiant  heat  to  pass  readily  through  them, 
and  are  called   diathermanous.      The  term  is   derived 
from  the  Greek  words  dia,  through,  and  thermos,  heat. 

If  a  plate  of  glass  be  held  up  before  an  iron  ball  heated 
to  dull  redness,  a  delicate  thermometer  held  behind  the 
plate  will  be  scarcely,  if  at  all,  affected.  If,  however,  a 
plate  of  rock  salt  be  put  in  place  of  the  glass,  the  ther- 
mometer rapidly  rises.  Rock  salt  is  the  most  diather- 
manous of  all  known  solids,  and  is  to  radiant  heat  what 
glass  is  to  light. 


1 74  HEAT. 

A  solution  of  iodine  in  bisulphide  of  carbon  is  wholly 
opaque  to  luminous  heat,  and  perfectly  diathermanous  to 
obscure  heat. 

251.  Obscure  Heat  always  accompanies  Luminous 
Heat.  —  If  a  luminous  beam,  as  that  from  the  lime  light, 
be  allowed  to  fall  upon  a  cell  with  glass  sides,  filled  with 
the  iodine  solution  (250),  all  the  luminous  heat  is  cut  off. 
Place  a  differential  thermometer  *  behind  the  cell  in  the 
dark  space,  and  we  find  that  a  beam  of  obscure  heat  is 
passing  through  the  cell.     Obscure  is  found  always  to 
accompany  luminous  heat ;    and  the  hotter  the  source, 
the  more  intense  are  the  obscure  rays.     This  may  be 
shown  by  heating  a  coil  of  platinum  wire  gradually  from 
dull  redness  to  a  white  heat,  and  then  cutting  off  the 
luminous  rays  by  the  iodine  cell,  and  letting  the  obscure 
rays  fall  on  a  differential  thermometer. 

252.  Heat  is  Reflected  and  Refracted  in  the  Same 
Way  as  Light.  —  Let  a  beam  of  light  fall  upon  a  mir- 
ror, and  hold   one    bulb  of  a    differential    thermometer 
in  its  path  after  it  has  been    reflected.      The    luminous 
heat  will  be  found  to  be  reflected  with  the  light.     Now 
cut  off  the  luminous  heat  with  the  iodine  solution,  and 
hold  the  bulb  again  in  the  path  which  the  reflected  beam 
took.     It  will  be  found  that  the  obscure  heat  has  also 
been  reflected  in  the  same  path  as  the  light. 

Let  the  luminous  beam  fall  upon  a  prism,  and  examine 
it  in  the  same  way  after  refraction.  It  will  be  found 
that  both  luminous  and  obscure  heat  are  refracted 
like  light. 

253.  tfeat  is  Dispersed  in  the  Same  Way  as  Light. 
—  Experiments  have  shown  that  radiant  heat  is  dispersed 
like  light  on  passing  through  a  prism  ;  and  that  obscure 
heat  is  less  refrangible  than  luminous  heat. 

*  A  thermometer  for  finding  the  difference  of  temperature  at 
two  points  (291). 


HEAT.  175 

254.  The   Spectrum  is  made  up  of  three  Parts.  — 
The  spectrum  is  found  to  be  made  up  of  three  parts :   (i) 
a  luminous  portion ;  extended,  at  the  red  end,  by  (2)  an 
obscure  thermal  portion ;  and,  at  the  violet  end,  by  (3) 
an  obscure  chemical  portion.     The  luminous  portion  is 
also  both  thermal  and  chemical. 

Black  lines  (called,  from  their  discoverer,  Fraun- 
hofer's  lines}  can  be  seen  crossing  the  luminous  part  of 
the  spectrum.  These  dark  lines  are  found  to  be  also 
cold  and  chemically  inactive.  Similar  cold  lines  are 
found  in  the  obscure  thermal  part. 

255.  Calorescence   and  Fluorescence. — If    a    jet   of 
mixed  hydrogen  and  oxygen  be  set  on  fire,  it  produces 
wh'at  is  called  the  oxy-hydrogen  flame.     This  flame  has 
very  little  light,  but  its  heat  is  intense.     The  radiations 
are  mainly  the  obscure  thermal  ones.     But  if  a  small 
cylinder  of  lime  be  put  into  the  flame,  the  light  becomes 
most  dazzling.      The  obscure  thermal  radiations  have 
been  changed  into  luminous  ones.    This  change  is  called 
calorescence. 

If  a  paper  washed  with  a  solution  of  quinine  be  held 
in  the  extreme  violet  end  of  the  spectrum,  the  obscure 
chemical  part  of  the  spectrum  becomes  luminous.  This 
change  of  the  obscure  chemical  rays  into  luminous  ones 
is  called  fluorescence. 

256.  Different    Solids,    Liquids,    and  Gases   absorb 
Heat  'with   different  Degrees  of  Readiness.  —  If  we 
cause  rays  of  heat  to  fall  upon  different  solids,  liquids, 
and  gases,  and  measure  with  a  delicate  thermometer  the 
heat  which  passes  through,  we  find  that  they  absorb  the 
same  kind  of  heat  very  differently. 

Again,  if  we  use  platinum  wire  (251)  as  a  source  of 
heat,  we  shall  find  that  glass  absorbs  obscure  heat  better 
than  luminous  heat ;  and,  in  general,  that  a  given  sub- 
stance absorbs  different  kinds  of  heat  in  different 


176  HEAT. 

proportions.  Among  gases  the  best  absorber  of  ob- 
scure heat  is  watery  vapor. 

257.  Good  Absorbers  are  good  Radiators.  —  Place  a 
heated  copper  ball  just  half-way  between  two  plates  of 
tin,  one  of  which  is  bright  and  the  other  coated  with 
lamp-black ;  and  hold  the  bulb  of  a  differential  ther- 
mometer against  the  back  of  each  plate.  The  coated 
plate  will  be  found  to  be  hotter  than  the  other,  showing 
that  it  is  the  better  absorberv 

Again,  place  the  plates  against  the  ball,  with  the 
coated  side  of  the  blackened  plate  outward ;  and  hold 
a  differential  thermometer  near  each  to  receive  the  heat 
radiated  by  it.  It  will  be  found  that  the  coated  plate 
is  the  better  radiator. 

Good  absorbers  always  prove  to  be  good  radiators. 


cfc 


SUMMARY. 


Heat  is  radiated  from  its  source  in  all  directions,  in 
straight  lines,  with  the  velocity  of  light.  (247,  248.) 

Radiated  heat  may  be  luminous  or  obscure.     (249.) 

Bodies  which  allow  heat  to  pass  readily  through  them 
are  called  diathermanous.  (250.) 

Radiant  heat  is  reflected,  refracted,  and  dispersed  in 
the  same  way  as  light.  (252,  253.) 

The  spectrum  is  made  up  of  three  parts:  (i)  a  lumi- 
nous part;  (2)  an  obscure  thermal  part;  and  (3)  an 
obscure  chemical  part.  (254.) 

Calorescence  is  the  change  of  the  obscure  thermal  rays 
into  luminous  rays.  Fluorescence  is  the  change  of  the 
obscure  chemical  rays  into  luminous.  (255.) 

Different  solids,  liquids,  and  gases  absorb  heat  very 
differently.  (256.) 

Good  absorbers  are  good  radiators.     (257.) 


HEAT. 


177 


EFFECTS  OF  HEAT  ON  BODIES. 

258.  The  Molecules  of  a  Body  transmit  Heat  to  one 
another.  —  When  one  end  of  a  poker  is  placed  in  the 
fire,  it  soon  becomes  red  hot,  and  the  heat  slowly  travels 
from  this  end  to  the  other.  This  heat  cannot  have  been 
radiated,  since  radiant  heat  travels  at  the  rate  of  190,000 
miles  a  second. 

Fig.  160. 


This  transmission  of  heat  from  molecule  to  molecule 
of  a  body  is  called  conduction. 

259.  Different   Solids  conduct  Heat  differently. — 
If  several  thermometer  bulbs  be   inserted  in  a  metallic 
rod,  as  shown  in  Figure  160,  and  one  end  of  the  bar 
be  heated,  the  mercury  will  begin  to  rise  in  the  ther- 
mometer nearest  the  heated  end,  and  then  in  the  others 
successively.     If  rods  of  other  metals  of  the  same  length 
and  thickness  be  tried  in  the  same  way,  it  will  be  found 
that   the   metals    differ   'widely   in    conductive  power. 
Those  which  are  good  conductors  of  heat  are  also  good 
conductors  of  electricity. 

260.  Liquids    and  Gases   are    Poor   Conductors   of 
Heat.  —  In  Figure  161,  a  differential  thefmometer  (251) 


12 


HEAT. 


Fis- l6x-  is   placed    in   a   glass   vessel    filled 

with  water.  Heat  is  applied  to  the 
surface  of  the  water  by  means  of  a 
dish  of  heated  oil.  If  the  water 
conducted  the  heat,  the  upper  bulb 
wTould  become  heated  sooner  than 
the  lower  one,  and  the  thermome- 
ter would  at  once  indicate  a  dif- 
ference of  temperature  between  the 
two  bulbs.  But  the  thermometer 
is  scarcely  affected. 

All   liquids    are   poor  conductors 
of  heat ;  and  gases  are  poorer  ones. 

261.  Heat    raises   the   Temperature    of  a  Body. — 
The  most  obvious  effect  of  the  heat  absorbed  by  a  body 
is  a   rise  of  temperature.     This  rise  is  indicated  by  the 
sense  of  touch,  but  more  accurately  by  a  thermometer. 

262.  A   Body  in  cooling  i°  gives  out  just  as  much 
Heat  as  it  takes  to  heat  it  l°.  —  Boil  half  a   pound  of 
water,  and  plunge  the  bulb  of  a   thermometer  into  it, 
and  it  will  indicate  a  temperature   of  212°.      Remove 
the  water  from  the  source  of  heat,  and  add  half  a  pound 
of  water  of  a  temperature  of  70°.      Stir  the  mixture  a 
short  time  with  the  bulb  of  a  delicate  thermometer,  and 
the   temperature  will  be  found  to  be    141°.     The   first 
half-pound  of  water  has  then  lost   Ji°  and  the  second 
has  gained  71° ;  in  other  words,  the  first  in  cooling  i° 
has   given    out   just   heat   enough    to  warm  the   second 
i°.     The  same  is  true  of  all  other  bodies. 

263.  It  requires  different  Amounts  of  Heat  to  raise 
the   Temperature   of  the   same    Weight    of  different 
Bodies  i°.  —  Heat  a  piece  of  tin  to   212°   by  plunging 
it  into  boiling  water,  and    then  plunge  it  into   its  own 
weight  of  water  at  70°.    The  resulting  temperature  will 
be  considerably  below  141° ;  showing  that  tin  in  cooling 


HEAT.  179 

i°  does  not  give  out  heat  enough  to  raise  the  water  i°. 
But  the  tin  in  cooling  i°  gives  out  just  as  much  heat  as  it 
takes  to  raise  its  temperature  i°.  Hence  it  takes  more 
heat  to  raise  the  temperature  of  a  pound  of  water  i°  than 
to  raise  that  of  a  pound  of  tin  i°.  If  copper  be  used 
instead  of  tin,  the  resulting  temperature  will  be  higher, 
but  still  below  141°.  It  requires,  then,  less  heat  to  raise 
the  temperature  of  a  pound  of  copper  i°  than  to  raise  that 
of  a  pound  of  water  i°,  but  more  than  it  takes  to  raise 
that  of  a  pound  of  tin  i°.  In  this  way,  wre  find  that  it 
takes  very  different  amounts  of  heat  to  raise  the  tem- 
perature of  the  same  weight  of  different  substances  i°. 

264.  Unit  of  Heat.  —  The  thermometer  indicates  the 
rise  of  temperature  in  a  body,  but  not  the  amount  of  heat 
required  to  raise  the  temperature.     It  is  therefore  desir- 
able to  have  some  -unit  by  which  the  heat  received  by  a 
body  may  be  expressed.     The  unit  usually  taken  is  the 
amount  of  heat  required  to  raise  the  temperature  of  a 
pound  of  water  i°.    A  unit  of  heat,  then,  is  the  amount 
of  heat  required  to  raise  the  temperature  of  one  pound 
of  water  i°. 

265.  Specific  Heat. —  The  amount  of  heat  required 
to  raise  the  temperature  of  a  pound  of  any  substance 
i°,  expressed  in  units,  is  called  the  specific  heat  of  that 
substance.     Thus  it  requires  -fa  of  a  unit  of  heat  to  raise 
the  temperature  of  one  pound  of  mercury  i  ° ;  and  the 
specific  heat  of  mercury  is  therefore  ^  or  .033. 

266.  Heat  causes  Solids  to  melt.  —  Place  a  dish  of 
water  at  the  temperature  of  32°  and  a  dish  of  ice  at  the 
same  temperature  side  by  side  in  a  warm  room,  and  hold 
a  thermometer  bulb  in  each.     The  temperature  of  the 
water  will  gradually  rise,  while  that  of  the  ice  will  not 
rise  until  the  whole  is  melted.      The  heat,  then,  which 
has  been  absorbed  by  the  ice  has  melted  it,  or  changed 
its  state. 


180  HEAT. 

The  second  effect  of  heat  upon  a  body,  then,  is  to 
change  its  state. 

267.  The  Melting-Points  of  different  Solids  are  very 
different.  —  Ice,  as  we  have  seen,  has  a  temperature  of 
32°.  Mercury  melts  at  —  40°;  and  alcohol  at  a  temper- 
ature lower  than  we  have  yet  been  able  to  produce.  On 
the  other  hand,  phosphorus  melts  at  111°  ;  iron  at  2012°; 
and  charcoal  at  a  higher  temperature  than  we  are  able 
to  produce.  The  melting-point  of  any  one  substance 
is,  under  the  same  circumstances,  always  the  same. 

Certain  bodies  become  soft  or  viscous  before  they 
melt.  Sealing-wax,  when  cold,  is  quite  brittle,  but 
when  heated  it  first  grows  plastic,  and  finally  melts. 
In  like  manner,  iron  before  melting  becomes  soft  in 
such  a  manner  that  pieces  may  be  easily  welded  to- 
gether or  moulded  into  any  form. 

26$.  Latent  Heat  of  Liquids.  —  If  a  pound  of  ice 
at  32°  be  mixed  with  a  pound  of  water  at  212°,  the  tem- 
perature, when  the  ice  is  melted,  will  be  50.5°.  It  has 
then  taken  161.5  units  of  heat  to  melt  a  pound  of  ice  and 
to  raise  its  temperature  from  32°  to  50.5°,  or  18.5°.  It 
therefore  takes  143  units  of  heat  to  melt  a  pound  of  ice. 
Heat  always  disappears  in  melting  a  solid ;  and  this 
heat  is  called  the  latent  heat  of  fusion,  or  the  latent 
heat  of  the  liquid,  since  it  is  concealed  in  the  liquid. 

By  the  latent  heat  of  a  liquid,  then,  we  mean  the  num- 
ber of  units  of  heat  required  to  melt  one  pound  of  the 
substance.  Thus  the  latent  heat  of  water  is  143  units, 
which  is  greater  than  that  of  any  other  liquid. 

When  the  liquid  passes  back  into  the  solid  state  again, 
its  latent  heat  reappears  as  sensible  heat. 

269.  Heat  causes  Liquids  to  boil.  —  Under  the  ordi- 
nary pressure,  if  water  be  raised  to  a  temperature  of 
212°,  it  begins  to  boil,  and  its  temperature  then  remains 
the  same  until  it  is  all  converted  into  steam.  The  heat, 


HEAT.  l8l 

then,  which  the  water  absorbs  changes  it  from  the  liquid 
to  the  gaseous  state.  Other  liquids  can  be  made  to  boil, 
but  at  very  different  temperatures.  Any  given  liquid, 
under  the  same  circumstances,  always  boils  at  the  same 
temperature. 

270.  Latent  Heat  of  Gases.  —  If  a  thermometer  be 
held  in  the  steam  just  over  boiling  water,  it  will   indi- 
cate a  temperature  of  212°.     Now,  as  water  is  receiving 
heat  all  the  time  it  is  boiling,  this  heat  must  be  latent  in 
the  steam.     The  latent  heat  of  different  gases  is  found  to 
vary  greatly.    The  latent  heat  of  steam  and  watery  vapor 
is  greater  than  that  of  any  other  gas  or  vapor,  hydrogen 
alone  excepted. 

271.  The  State  of  a  Body  depends  upon  its  Temper- 
ature.—  When  a  solid  is  heated,  its  temperature  rises 
till  it  reaches  the  melting-point,  where   it  remains  sta- 
tionary until  the   solid  is  melted.      It  then  rises  again 
until  it  reaches  the  boiling-point,  where  it  again  remains 
stationary  until  the  liquid  is  converted  into  a  gas.    When 
a  gas  is  sufficiently  cooled,  it   goes   through  the  same 
changes  in  the  reverse  order. 

It  is  because  different  substances  have  very  different 
boiling-points  that  they  can  exist  in  nature,  some  as 
solids,  some  as  liquids,  and  some  as  gases. 

272.  The  Boiling- Point  of  Water  falls  as  the  Pres- 
sure on  its  Surface  diminishes.  —  Fill  a  flask  two-thirds 
full  of  water,  boil  it  for  some  time,  cork  it  tightly,  remov- 
ing it  at  the  same  time  from  the  source  of  heat,  and 
invert  it,   as  shown  in   Figure   162.      Pour   cold  water 
upon  the  flask,  and  it  begins  to  boil  again. 

At  first  the  upper  part  of  the  flask  is  full  of  steam, 
whose  elastic  force  causes  it  to  press  upon  the  water. 
When  cold  water  is  poured  upon  the  flask,  this  steam  is 
condensed,  the  pressure  is  diminished,  and  the  water 
boils,  though  at  a  lower  temperature. 


182 


HEAT. 


Fig.  162. 


The  height  of  a  mountain  can  be  estimated  quite  ac- 
curately from  the  dif- 
ference between  the 
boiling-points  at  its 
summit  and  at  its  base. 
Steam  occupies  very 
much  more  space  than 
the  same  weight  of  wa- 
ter, and  there  is  no 
cohesion  among  its 
molecules  (22).  When, 
therefore,  water  Boils, 
both  the  cohesion  of  the 
liquid  and  the  pressure 
of  the  atmosphere  must 
be  overcome,  since  both 
of  these  tend  to  keep 
the  molecules  together. 
Hence,  when  either  the 
cohesive  force  or  the 

external  -pressure    is   changed,  the  boiling-point  will 

also  change. 

273.    The  Spheroidal  State.  —  If  two  or  three  drops 

of  water  be  poured  into   a   red-hot   metallic  cup,  they 

gather  into  a  globule,  which  runs  about  without  boiling. 

The  water  is  now  said  to  be  in  the  spheroidal  state. 

Fig.  163. 


Turn  a  cup  c  bottom  up  (Figure  163),  heat  it  to  red- 
ness, and  carefully  put  a  drop  of  water  d  upon  it  with  a 


HEAT.  183 

dropping-tube.  Place  behind  the  drop  a  platinum  wire 
a  b,  heated  to  a  white  heat  by  a  battery.  With  the  eye 
at  e,  the  platinum  wire  can  be  seen  between  the  drop  and 
the  cup,  showing  that  the  drop  does  not  touch  the  cup. 

As  soon  as  the  drop  comes  near  the  heated  cup,  steam 
is  generated  beneath  it,  and  acts  like  an  elastic  spring 
to  lift  the  drop  from  the  surface.  As  the  cup  cools, 
this  spring  gives  way,  and  the  water,  on  touching  the 
surface,  is  suddenly  converted  into  steam.  Boiler  ex- 
plosions are  probably  often  caused  by  this  sudden  change 
from  the  spheroidal  state  to  steam. 

274.  Evaporation.  —  If  water  is  exposed  in  an  open 
vessel  at  the  ordinary  temperature,   it  gradually  disap- 
pears, passing  off  in  the  form  of  vapor.     This  vapor  is 
formed  slowly  and  only  at  the  surface;  while,  in  boil- 
ing, steam  is  formed  rapidly  and  throughout  the  liquid. 
Water  is  thus  evaporated  into  the  atmosphere  at  all  tem- 
peratures, but  more  rapidly  as  the  temperature  rises. 

275.  Condensation. — A  gas  condenses  at  the  same 
point  at  which  its  liquid  boils;  and,  as  pressure  raises 
the  boiling-point,  it  also  raises  the  point  at  which  a  gas 
will  condense.     Under  the  combined  action  of  pressure 
and  cold,  almost  every  known  gas  has  been  liquefied. 

276.  Freezing- Mixtures. — When  a  solid  melts,  or  a 
liquid  evaporates,   a  large  amount  of  heat  is  rendered 
latent.     Advantage  is  taken  of  this  fact  to  obtain  an  arti- 
ficial reduction  of  temperature.     One  of  the  most  com- 
mon freezing-mixtures  is  composed  of  salt  and  pounded 
ice.     The   substance  to  be  frozen  is  placed  in  a  small 
vessel  which  is  put  in  a  larger  one  and  packed  round 
with  this  mixture.      The  ice  rapidly  melts,  and  in  doing 
so  absorbs  a  large  amount  of  heat,  thus  reducing  the 
temperature  of  the  inner  vessel. 

A  much  greater  degree  of  cold  is  obtained  by  the  rapid 
evaporation  of  a  liquid  than  by  the  melting  of  a  solid. 


184  HEAT. 

If  solid  carbonic  acid  be  mixed  with  ether,  it  evapo- 
rates very  rapidly.  By  means  of  such  a  mixture,  20  or  30 
pounds  of  mercury  may  be  readily  frozen.  If  the  mixt- 
ure be  placed  under  an  exhausted  receiver,  the  evapora- 
tion is  greatly  quickened.  Faraday  thus  reached  a  tem- 
perature of  —  1 66°  F. 

277.  Solids  are   expanded  by  Heat;    but  different 
Solids  expand  unequally  for  the  same  Rise  of  Tem- 
perature. —  We  have  already  learned  (6)  that  solids  are 
expanded  by  heat.     If  now  a  bar  of  iron  and  one  of  cop- 
per be  riveted  together  and  then  plunged  in  boiling  water, 
so  that  the  temperature  of  both  may  be  raised  to  the  same 
point,  the  compound  bar  will  become  curved,  the  copper 
being  the  convex  side.     This  is  because  copper  is  ex- 
panded more  than  iron  for  the  same  rise  of  temperature. 
Scarcely  any  two  solids  are  expanded  alike  by  heat. 

278.  Liquids  are  expanded  by  Heat;  but  different 
Liquids  expand  unequally  for  the  same  Rise  of  Tem- 
perature.—  Fill  a  test-tube  with  water,  and  then  close  it 
with  a  rubber  cork  through  which  passes  a  fine  glass 
tube.      Plunge   the  test-tube  in   boiling  water,   and  the 
liquid  will  rise  in  the  tube ;   showing  that   it  has  been 
expanded  by  heat. 

Fill  a  second  test-tube  with  alcohol,  and  plunge  both 
into  boiling  water.  The  alcohol  will  rise  higher  in  the 
tube  than  the  water  will,  showing  that  it  is  expanded 
more  by  the  heat.  Different  liquids,  then,  expand  un- 
equally for  the  same  rise  of  temperature. 

279.  Gases  are   expanded   by   Heat,   and  different 
Gases  expand  equally  for  the  same  Rise  of  Temper- 
ature. —  Close  a  pint  flask  with  a  cork  through  which 
passes  a  bent  tube,  and  connect  the  tube  with  a  jar  in- 
verted over  water.     Plunge  the  flask  into  boiling  water, 
and  bubbles  of  air  rush  over  into  the  jar.     All  gases  are 
thus  expanded  by  heat. 


HEAT. 


'85 


Fill  now  the  same  flask  with  hydrogen,  oxygen,  or  any 
other  gas ;  connect  it  with  the  same  jar  as  before,  and 
again  plunge  the  flask  into  boiling  water.  Precisely  the 
same  amount  of  gas  will  pass  over  as  at  first. 

Solids,  liquids,  and  gases  are  expanded  by  heat; 
solids  and  liquids  unequally,  and  gases  equally,  for 
the  same  rise  of  temperature. 

280.  Convection.  —  Since  the  molecules  of  liquids  and 
gases  are  free  to  move,  their  expansion,  when  they  are 
heated  unequally  in  different  parts,  will  create  currents. 
The  unexpanded  and  heavier  portions  will  tend  to  dis- 
place the  lighter  ones  and  to  compel  them  to  rise.     As 
these  heavier  portions  become  heated,  they  in  turn  tend 
to  rise  and  give  place  to  colder  portions ;  and  so  on. 

These  currents  tend  to  distribute  the  heat,  and  this 
mode  of  distribution  is  called  convection.  J - 

281.  Convection  of  Liquids.  —  In  Figure  164,  we  have 
a  glass  beaker  filled  with  water  heated  by  a  lamp  below. 
A  little  sawdust  is  added  to  the 

water,  and  its  motions  show 
that  a  current  is  passing  up  the 
centre  of  the  vessel  and  down 
at  the  sides,  as  indicated  by  the 
arrows  in  the  figure.  Each 
molecule  is  thus  seen  to  come 
to  the  bottom  to  get  heated, 
and  then  to  return  to  the  sur- 
face. It  is  in  this  way  that 
water,  which  is  a  bad  conduc- 
tor, is  readily  heated  when  the 
heat  is  applied  below. 

282.  Oceanic      Currents.  — 
Oceanic  currents  are  produced 

by  convection.      The    temperature    of    the    sea    in    the 
tropics  is  about  50°  higher  than  at  the  poles,  and  the 


Fig.  164. 


1 86  HEAT. 

specific  gravity  of  the  water  is  therefore  muth  less.  To 
restore  the  equilibrium,  the  warmer  and  lighter  water 
of  the  tropical  regions  flows  towards  the  poles,  and 
the  colder  and  denser  water  of  the  polar  regions  flows 
towards  the  equator.  If  the  whole  earth  were  covered 
with  water  of  the  same  saltness,  we  should  everywhere 
have  a  surface-current  from  the  equator  towards  the 
poles,  and  an  under-current  from  the  poles  towards 
the  equator.  But  owing  to  the  obstructions  offered  by 
the  land  and  by  the  inequalities  in  the  bed  of  the 
ocean,  and  to  the  different  degrees  of  saltness,  and 
therefore  of  density,  in  different  parts  of  the  sea,  these 
two  great  currents  are  broken  up  into  innumerable  minor 
currents  and  counter-currents. 

The  most  remarkable  of  these  currents  is  the  Gtilf 
Stream,  which  issues  from  the  Gulf  of  Mexico,  and, 
crossing  the  Atlantic  in  a  north-easterly  direction,  washes 
the  western  coast  of  Europe. 

283.  Convection  of  Gases.  —  If  a  lighted  candle  be 
held  in  the  crack  of  a  door  which  opens  from  a  warm 
into  a  cold  room,  the  flame  will  be  blown  outward  at  the 
top  of  the  door  and  inward  at  the  bottom,  while  half-way 
up  it  will  burn  steadily.  A  current  of  cold  air  is  passing 
into  the  room  at  the  bottfem,  driving  out  a  current  of 
warm  air  at  the  top. 

It  is  mainly  by  convection  that  the  air  in  a  room  is 
heated.  The  air  near  the  stove  is  heated  and  expanded, 
and  then  forced  upward  by  the  current  of  colder  air. 

When  a  building  is  heated  by  a  furnace,  this  is  placed 
in  the  cellar  and  encased  in  brick-work  or  in  sheet  iron. 
The  space  between  the  fire-pot  and  the  casing  is  con- 
nected by  the  air-box  with  the  outer  atmosphere,  and 
by  flues  or  pipes  with  the  rooms  to  be  heated.  The 
air  about  the  fire-pot  first  becomes  heated,  and  is  driven 
up  through  the  pipes  by  the  cold  air  from  without. 


HEAT.  187 

284.  The  Relation  of  Water  to  Heat  and  Climate. 
—  Water,  from  its  high  specific  and  latent  heat,  has  a 
marked  influence  on  climate.      It  makes  the  transition 
from  winter  to  summer  and  from  summer  to  winter  more 
gradual.     In  the  spring,  when  the  snow  begins  to  melt,  a 
large  amount  of  heat  is  absorbed  from  the  air  and  ren- 
dered latent.    After  the  snow  and  ice  are  all  melted,  such 
is  the  specific  heat  of  water  that  it  requires  a  great  deal 
of  heat  to  raise  its  temperature.     In  the  fall,  on  the  other 
hand,  as  the  water  cools  down  and  freezes,  it  gives  out 
all  the  heat  which  it  had  absorbed  in  the  spring. 

285.  The  Irregular  Expansion  and  Contraction  of 
Water.  —  If  water  at  the  temperature  of  39°  be  either 

warmed  or  cooled,  it  expands.  This  temperature  is 
hence  called  the  point  of  maximum  density  of  water. 

We  can  now  understand  why  water  often  bursts  the 
pipe  or  vessel  in  which  it  freezes.  Water  is  the  only 
liquid  'which  has  such  a  point  of  maximum  density, 
and  there  are  but  very  few  substances  which  expand 
when  they  become  solid.  Iron  is  one,  and  it  is  owing 
to  this  property  that  it  is  so  well  adapted  for  castings. 
As  it  solidifies,  it  expands  so  as  completely  to  fill  the 
mould. 

This  irregular  expansion  of  water  is  of  the  greatest 
importance.  Before  freezing  it  begins  to  grow  lighter, 
so  that  the  freezing  begins  at  the  surface ;  and  the  ice, 
being  lighter  still  and  also  a  poor  conductor  of  heat, 
floats  upon  the  water  and  keeps  it  from  freezing  very 
deep.  If  water  continued  to  contract  as  it  cooled,  it 
would  begin  to  freeze  at  the  bottom,  and  during  the 
winter  our  lakes  and  rivers  would  become  solid  masses 
of  ice.  This  would  be  fatal  to  all  animal  life  in  the 
water ;  and,  as  water  is  a  very  poor  conductor  of  heat, 
it  would  melt  only  to  the  depth  of  a  few  feet  during 
the  summer. 


1 88  HEAT. 

286.  Heating  by  Steam.  —  It  is  now  quite  common 
to  warm  buildings  by  steam.     Pipes  run  from  the  boiler 
through  the  rooms  to  be  heated,  and    then  back  to  the 
boiler   again.      The   steam  passes   from   the  boiler  into 
these    pipes,  where    it   is  condensed    and    runs  back  as 
water  to    the  boiler.      Now  every  pound  of  water  con- 
verted into  steam  in  the  boiler  takes  up  over  900  units 
of  heat,  and  every  pound  of  steam  which  condenses  in 
the  pipes  gives  out  the  same  amount  of  heat  into  the 
rooms.     The  water  thus  acts  as  carrier  of  heat  between 
the  furnace  and  the  room  where  it  is  wanted. 

287.  A  Hot-House  is  a  Tra£  t°  catch  Sunbeams. — 
A  hot-house  is  covered  with  glass.     Now  we  have  seen 
that  luminous  heat,  like  that  from  the  sun,  passes  readily 
through   glass,  while    obscure    heat    can    scarcely   pass 
through    it    at   all.      As,    therefore,    the    sunbeams  beat 
down  upon  the  roof  of  a    hot-house,  they  easily  pene- 
trate   the    glass ;    but,  when   they  fall    upon    the    plants 
and  furniture  within,  they  are  radiated  back  as  obscure 
heat,  and    are    unable    to    make    their  way  through  the 
glass  except  by  the  slow  process  of  conduction.      It  is 
because  the  sunbeams  are  thus  caught  in  a  trap,  that  the 
temperature  inside  a  hot-house  on  a  sunny  day  is  always 
higher  than  outside. 


SUMMARY. 

Heat  is  transmitted  from  molecule  "to  molecule  of 
bodies  by  conduction.  Liquids  and  gases  are  poor  con- 
ductors. (258,  260.) 

The  heat  which  a  body  absorbs  is  partially  used  in 
raising  its  temperature.  (261.) 

A  body  in  cooling  i°  gives  out  just  as  much  heat  as  it 
takes  to  warm  it  i°.  (262.) 


HEAT.  189 

It  takes  a  different  amount  of  heat  to  raise  the  tem- 
perature of  the  same  weight  of  different  substances  i°. 

The  amount  of  heat  required  to  raise  the  temperature 
of  one  pound  of  water  i°  is  called  a  unit  of  heat. 

The  amount  of  heat  required  to  raise  the  temperature 
of  one  pound  of  any  substance  i°,  expressed  in  thermal 
units,  is  called  its  specific  heat.  (263-265.) 

The  heat  which  a  body  absorbs  is  sometimes  used  in 
changing  its  state.1  (266.) 

The  melting  and  boiling  points  are  the  same  for  the 
same  substance  under  the  same  pressure  ;  but  those  of 
different  substances  are  different.  (267,  269.) 

When  a  substance  melts  or  boils,  a  certain  definite 
amount  of  heat  is  rendered  latent. 

The  latent  heat  of  water  is  higher  than  that  of  any 
other  liquid  ;  and  that  of  steam  is  higher  than  that  of 
any  other  vapor.  (268,  270.) 

The  boiling-point  of  water  is  raised  by  increasing  the 
pressure.  (272.) 

When  water  is  put  into  a  red-hot  vessel,  it  is  prevented 
.  from  coming  in  contact  with  the  heated  surface  by  a 
layer  of  steam,  and  is  said  to  be  in  the  spheroidal 
state.  (273.) 

Liquids  evaporate  at  all  temperatures,  but  more  rapidly 
as  the  temperature  rises.  Vapors  condense  at  the  same 
point  as  that  at  which  their  liquids  boil.  (274,  275.) 

The  heat  absorbed  by  a  body  is  used  partially  in 
pushing  the  molecules  apart,  or  expanding  it.  Dif- 
ferent solids  and  liquids  expand  unequally,  and  different 
gases  equally,  for  the  same  rise  of  temperature.  (277- 

279-) 

When  a  gas  or  a  liquid  is  heated  beneath  its  surface, 
currents  are  produced  which  distribute  heat  by  convec- 
tion. It  is  in  this  way  that  the  Gulf  Stream  and  other 
oceanic  currents  are  produced.  (280-283.) 


190  HEAT. 

Water,  by  its  high  specific  and  latent  heat,  tends  to 
make  the  change  of  seasons  more  gradual.  Its  irregu- 
lar expansion  and  contraction  prevent  the  lakes  and 
rivers  from  freezing  solid  in  the  winter.  (284,  285.) 


THERMAL  INSTRUMENTS. 

288.  The  Mercurial  Thermometer.  —  The  common 
thermometer  is  used,  as  its  name  implies,  to  measure 
temperature. 

In  making  a  thermometer,  the  bulb  and  tube  are  first 
filled  with  mercury.  The  mercury  is  then  heated  up  to 
the  highest  temperature  which  the  thermometer  is  in- 
tended to  measure,  and  the  end  of  the  tube  is  sealed 
air-tight  by  melting  the  glass.  As  the  bulb  cools  again, 
the  mercury  falls  in  the  tube,  leaving  a  vacuum  above  it. 

On  the  thermometer  scale  there  are  two  fixed  points : 
that  at  ivhich  ice  melts,  or  the  freezing-point;  and 
that  at  which  water  boils,  or  the  boiling-point. 

The  freezing-point  is  found  by  plunging  the  bulb  into 
melting  ice,  and  noting  the  position  of  the  mercury  in 
the  tube.  Melting  ice  is  used  rather  than  freezing  water, 
because  it  is  found  that  if  water  be  kept  perfectly  still  it 
can  be  cooled  several  degrees  below  the  freezing-point 
before  it  congeals ;  while  ice,  at  the  ordinary  pressure, 
always  melts  at  the  same  temperature. 

The  boiling-point  of  water  is  not  always  the  same 
(272)  ;  but  its  steam  always  has  the  same  temperature 
at  the  ordinary  pressure.  The  boiling-point  is  therefore 
found  by  enclosing  the  bulb  and  tube  in  a  steam-bath. 

On  the  Fahrenheit  scale,  which  is  the  one  in  common 
use  in  this  country  and  England,  the  freezing-point  is 
marked  32,  and  the  boiling-point  212;  the  space  be- 
tween the  two  being  divided  into  180  equal  parts,  or 
degrees.  These  equal  divisions  are  continued  above  the 


HEAT. 


boiling-point  and  below  the  freezing-point;  but  not  be- 
low—  38°  nor  above  576°,  since  mercury  freezes  at  the 
former  point  and  boils  at  the  latter. 

On  the  Centigrade  scale,  which  is  the  one  commonly 
used  in  France,  and  the  one  generally  preferred  by  scien- 
tific men,  the  freezing-point  is  marked  o,  and  the  boil- 
ing-point 100.  5°  of  this  scale,  then,  are  equal  to  9°  of 
the  Fahrenheit  scale. 

A  third  scale,  known  as  Reaumur's,  is  in  general  use 
in  Germany.  On  this  scale  the  freezing-point  is  marked 
o,  and  the  boiling-point  80. 

289.  The  Alcohol    Thermometer.  —  When   tempera- 
tures below  — 38°  are  to  be  measured,   alcohol   is  used 
instead   of  mercury.     An    alcohol    thermometer   is   not, 
however,  so  accurate  as  a  mercurial  one. 

290.  The   Air    Thermometer.  —  There    are   various 
ways  of  measuring  temperatures  above  the  boiling-point 
of  mercury,   but  the  best  is  by  means  of  the  air  ther- 
mometer.    The  expansive  force  of  air  is  very  regular 
for  all  known  temperatures,  but  it  expands  so  rapidly 
that    to    measure    the   ordinary   range   of    temperatures 
would  require  too  long  a  tube. 

The  expansion  of  the  air  in  the 
tube  is  indicated  by  the  movement  of 
a  column  of  liquid  upon  which  it  acts. 

291.  The  Differential  Thermome- 
ter. —  This  instrument  shows  the  dif- 
ference in  temperature  between  two 
neighboring  substances  or  places.    In 
the  one   invented    by  Leslie   (Figure 
165),  two  bulbs,  A  and  B,  filled  with 
air,   are    connected    by  a    bent   tube. 
A  little  colored  liquid  fills  the  lower 
part   of   this   tube,    and   rises   to   the 
levels  C  and  D  when  both  bulbs  are 


Fig.  165. 


I92  HEAT. 

of  the  same  temperature.  But  should  A  become  warmer 
than  £>,  since  air  expands  very  much  for  an  increase  of 
temperature,  the  column  of  liquid  will  be  pushed  down 
at  C  and  made  to  rise  at  JD;  and  this  motion  will  be 
reversed  when  B  becomes  warmer  than  A.  The  slightest 
difference  of  temperature  is  therefore  indicated  at  once. 
Sulphuric  acid,  or  some  other  liquid  which  is  not  volatile, 
is  used  in  the  tube. 

292.  Effect  of  Temperature  upon  Measures  of  Time. 
—  The  rate  of  a  clock  depends  upon  the  time  in  which 
its  pendulum  vibrates,  and  that  of  a  watch  upon  the  time 
in  which  its  balance-wheel  oscillates.  Now  since  the 
change  of  temperature  alters  the  length  of  a  pendulum, 
it  likewise  alters  its  time  of  vibration  (106).  The  higher 
the  temperature,  the  more  slowly  does  it  vibrate.  In 
like  manner,  a  change  of  temperature,  by  altering  the 
dimensions  of  the  balance-wheel  of  a  watch  and  the 
force  of  the  spring,  will  cause  it  to  vibrate  more  slowly 
in  hot  weather  than  in  cold. 

Fig.  166.  293.  Graham's  Mercurial  Pendulum.  —  The 
first  attempt  to  compensate  for  change  of  length 
in  a  pendulum  was  made  by  Graham,  an  English 
clockmaker.  The  rod  of  his  pendulum  (Figure 
1 66)  was  made  of  glass,  to  the  lower  end  of  which 
was  attached  a  cylindrical  vessel  containing  mer- 
cury. As  the  glass  rod  expands  by  heat,  the 
bottom  of  the  vessel  which  contains  the  mercury 
will  of  course  be  carried  farther  from  the  point 
of  suspension ;  but  since  the  mercury  resting  on 
this  base  expands  upwards,  its  centre  of  gravity 
is  raised,  or  brought  nearer  the  point  of  suspen- 
sion. The  lowering  of  the  centre  of  gravity, 
due  to  the  expansion  of  the  glass,  may  thus  be 
counteracted  by  the  rise  of  the  same,  due  to  the 
expansion  of  the  mercury. 


HEAT.  193 

294.  Compensation  Balance  -  Wheel.  —  The  balance- 
wheel  of  a  watch  is  sometimes  made,  as  in  Figure  167, 
not  with  one  continuous  rim,  but  with  pig.  167. 

a  broken  rim  of  several  separate 
pieces,  all  of  which  are  fixed  at  one 
end  and  free  at  the  other,  the  free 
ends  being  loaded ;  and  each  piece 
is  composed  of  two  metals,  of  which 
the  more  expansible  is  placed  outside. 
On  a  rise  of  temperature,  then,  the 
loaded  ends  'will  approach  the  centre.  This  is  made  to 
counteract  the  effect  produced  on  the  rate  of  the  watch 
by  the  expansion  of  the  wheel,  which  carries  the  cir- 
cumference farther  from  the  centre. 

295.  Other  Effects  of  Expansion.  —  The  force  ex- 
erted by  solids  in  contracting  or  expanding,  or  by  liquids 
in  expanding,  is  very  great.     If  a  strong  vessel  be  en- 
tirely filled  with  a  liquid  and  then  sealed  tightly,  the 
vessel  will  burst  if  considerably  heated. 

In  the  arts  it  is  of  great  importance  to  bear  in  mind 
the  intensity  of  this  force,  sometimes  with  the  view  of 
guarding  against  its  action,  and  sometimes  in  order  to 
make  it  useful.  Thus,  bars  of  furnaces  must  not  be 
fitted  tightly  at  their  extremities,  but  must  at  least  be  free 
at  one  end.  In  making  railways,  also,  a  small  space 
must  be  left  between  the  successive  rails.  For  a  similar 
reason  water-pipes  and  gas-pipes  are  fitted  to  each  other 
by  telescopic  joints. 

296.  Wet  and  Dry  Bulb  Hygrometer.  —  A  hygrome- 
ter is    an  instrument  for   measuring  the   amount  of 
moisture  in  the  air.     The  one  invented  by  Mason  con- 
sists of  two  thermometers   (Figure  168)  placed  side  by 
side,  one  having  a  dry  bulb  and  the  other  a  bulb  covered 
with  muslin,  kept  moist  by  means  of  a  string  dipping  in 
water.     The  wet  bulb  is  chilled  by  the  evaporation  of 

13 


HEAT. 


• l68-  the  water  from  it,  since  this 

evaporation  renders  some  of 
its  heat  latent.  The  drier 
the  air,  the  more  rapid 
the  evaporation,  and  the 
greater  the  difference  be- 
tween the  readings  of  the 
two  thermometers. 

297.  Edson's  Hygrodeik. 
—  This  is  an  improved  form 
of  Mason's  hygrometer.  It 
differs  from  all  other  hygro- 
meters in  having  a  dial  and 
pointer,  showing  at  a  glance 
the  temperature,  the  degree 
of  humidity,  the  absolute 
amount  of  vapor  in  each 
cubic  foot  of  air,  and  the 
dew-point. 

The  dew-point  is  the  tem- 
perature at  which  the  moisture  of  the  air  begins  to  be 
deposited  as  dew. 


SUMMARY. 

The  thermometer  is  used  to  measure  temperature. 
The  thermometer  scales  most  used  are  Fahrenheit's, 
the  Centigrade,  and  Reaumur's.  (288-290.) 

The  differential  thermometer  serves  to  measure  the 
difference  of  temperature  at  two  places.  (291.) 

The  expansive  power  of  heat  may  be  made  to  regu- 
late the  rate  of  clocks  and  watches.  (292-294.) 

The  hygrometer  and  hygrodeik  are  instruments  for 
measuring  the  amount  of  moisture  in  the  air.  (296,  297.) 


ELECTRICITY. 


MAGNETISM. 

298.  Magnets.  —  If  we  bring  one  end  of  an  ordinary 
bar  magnet  in  contact  with  a  pile  of  iron  tacks,  we  find 
that  some  of  the  tacks  cling  to  the  magnet.      The  force 
residing  in  a  magnet  and  shown  by  its  attracting  iron 
is  called  magnetism. 

There  is  a  certain  iron  ore  which  has  the  power  of 
attracting  iron.  This  ore  seems  to  have  been  first  found 
near  Magnesia,  a  city  of  Asia  Minor ;  hence  the  name 
magnet.  Natural  magnets  are  called  loadstones  (more 
properly  lodestones),  that  is,  stones  that  lead  or  draw 
iron. 

299.  The  Power  of  a  Magnet  resides  chiefly  at  the 
Ends.  —  If  a  small  iron  ball,  suspended  by  a  string,  be 
moved  alongside  a  bar  magnet,  it  is  scarcely  attracted  at 

Fig.  169. 


the  middle  of  the  bar.  As  it  approaches  either  end,  it  is 
attracted  more  and  more,  and  near  the  ends  the  attrac- 
tion is  much  the  strongest. 


196  ELECTRICITY. 

Lay  a  piece  of  stiff  drawing-paper  upon  a  strong 
magnetic  bar,  and  strew  fine  iron-filings  over  it.  The 
particles  of  iron  (Figure  169)  arrange  themselves  in 
lines  radiating  from  the  poles,  called  lines  of  mag- 
netic force,  or  magnetic  curves. 

300.  The  Forces  at  the  Ends  of  a  Magnet  act  in 
Opposite    Directions.  —  Suspend    a   bar    magnet    by  a 
string  (Figure  170)  so  that  it  can  turn  freely.      Bring 

Fig.  170.  one  end  of  a  bar  magnet  near  one 

end  of  the  suspended  magnet,  and 
the  latter  is  drawn  towards  it.  Re- 
verse the  ends  of  the  bar  magnet, 
and  the  end  of  the  suspended  mag- 

A ^r    net  is  repelled.      This  shows   that 

the  forces  at  the  ends  of  a  magnet 
act  in  opposite  directions. 

The  ends  of  the  magnet,  or  the  points  where  the 
opposite  forces  reside,  are  called  poles. 

301.  The  Magnetic  Needle.  —  A  bar  magnet  poised 
or  suspended  so  as  to  turn  freely  is  called  a  magnetic 
needle.      One   of  its  poles   will  always  point   to   the 
north,    and   is   called   the    north  pole.      The    opposite 
pole  is  called  the  south  pole. 

302.  The  Earth  acts  like  a   Magnet.  —  If  a  small 
needle  which   is   free   to   move   horizontally  be   placed 
upon  a  bar  magnet,   its    south    pole  will  always    point 
towards  the  north  pole  of  the  latter.      If  a  small  dip- 
ping needle,  that  is,  a  needle  which   is  free  to  move 
vertically,  be  placed  above  the  middle  of  a  bar  magnet, 
it  stands  parallel  with  the  bar  magnet.     If  it  be  moved 
towards  the  north  pole  of  the  magnet,  the  south  pole 
dips   more   and   more    towards    the    magnet.      If  it  be 
moved  from  the  centre  of  the  bar  magnet  towards  the 
south  pole,  its  north  pole  dips  in  the  same  way. 

Now,  a  magnetic  needle  (301)  points  north  and  south 


ELECTRICITY. 


I97 


when  held  above  the  earth.  A  dipping  needle  near  the 
equator  stands  horizontally ;  when  carried  north  from 
the  equator,  its  north  pole  dips  towards  the  horizon ; 
and  when  carried  south  from  the  equator,  its  south 
pole  dips. 

We  see,  then,  that  the  earth  acts  upon  a  magnetic 
needle  like  a  magnet  'whose  poles  are  near  the  poles  of 
the  earth. 

303.  Like  Poles  of  Magnets  repel  and  unlike  Poles 
attract   each   other.  —  Bring    the    north   pole  of  a  bar 
magnet  near  the  north  pole  of  a  needle,  and  the  latter 
will  be  repelled.     Bring  the  south  pole  of  this  magnet 
near  the  north  pole   of  the   needle,   and  it  will  be  at- 
tracted.    The  experiment  with  the  bar  magnet  and  the 
dipping  needle  (302)  also  illustrates  this  law. 

304.  Magnetism   is   developed  in  Iron   or  Steel  by 
Induction.  —  When  a  piece  of  soft  iron  is  brought  in 
contact  with  the  pole  of  a  magnet,  it  will  attract  other 
pieces  of  iron,   showing  that   magnetism  is  developed 
in  the  iron  by  contact  'with  the  magnet.     Magnetism 
can  be  developed,  or  induced,  in  a  piece  of  steel  in  the 
same  way.      The  iron  loses    its  magnetism  as  soon  as 
it  is  taken    away  from    the   magnet,  'while   the   steel 
retains  it.     The  iron  or  steel  need  not  come  in  actual 
contact   with    the    magnet.      Magnetism  Fig.  171. 
will   be    induced    in    it,    if  it   merely  be 

brought  very  near  the  pole. 

305.  Forms    of  Magnets.  —  Ordinary 
magnets     are     made     of   steel.      When 
straight,  they  are    called   bar  magnets; 
when  bent  into  the  shape  of  the  letter 
U,  they   are    called    horseshoe   magnets. 
Several  bar  or  horseshoe  magnets  con- 
nected (Figure    171)  constitute    a    mag- 
netic battery. 


198  ELECTRICITY. 


SUMMARY. 

The  force  which  enables  a  magnet  to  attract  iron  is 
called  magnetism.  (298.) 

This  force  resides  chiefly  at  the  ends,  or  poles,  of  a 
magnet.  It  radiates  from  these  poles  in  curved  lines, 
called  lines  of  magnetic  force,  or  magnetic  curves. 
(299,  300.) 

The  forces  at  the  poles  of  a  magnet  act  in  opposite 
directions.  (300.) 

The  earth  acts  upon  a  needle  like  a  magnet.  Its 
magnetic  poles  are  situated  near  the  poles  of  its  axis. 


Like  poles  of  magnets  repel,  and  unlike  poles  attract, 
each  other.  (303.) 

A  magnet  can  develop  magnetism  in  iron  or  steel  by 
induction.  Soft  iron  loses  its  magnetism  as  soon  as  it  is 
withdrawn  from  the  influence  of  the  magnet,  while  steel 
retains  its  magnetism  permanently.  (304.) 


VOLTAIC   ELECTRICITY. 


306.  The  Voltaic  Pair.  —  If  a  strip  of  amalgamated 
zinc  (zinc  which  has  been  immersed  in  mercury)  and 
another  of  copper  be  placed  in  a  cup  of  dilute  sulphuric 
acid,  no  action  takes  place  so  long  as  the  zinc  and  copper 
are  not  connected.  If  we  join  the  plates  by  means  of  a 
wire,  bubbles  of  hydrogen  gas  at  once  appear  at  the  cop- 
per plate,  and  the  acid  begins  to  dissolve  the  zinc  plate. 

If  a  small  magnetic  needle  be  held  near  the  wire  when 
the  plates  are  connected  (Figure  172),  the  needle  will 


ELECTRICITY. 


199 


Fig.  172. 


be  turned  aside,  showing  that  there  is 
some  new  force  in  the  wire.  This 
new  force  is  called  electricity;  and,  as 
it  seems  to  flow  through  the  wire,  it  is 
called  in  this  case  the  electric  current. 
It  is  produced  by  the  chemical  action 
between  the  acid  and  the  zinc. 

Two  plates  thus  connected  form  a 
voltaic  pair  or  cell.  The  zinc  is 
called  the  active  plate,  and  the  copper 
the  passive  plate.  The  passive  plate 
is  often  made  of  other  substances. 

The  active  plate  is  also  called  the  negative  pole,  and 
the  passive  plate  the  positive  pole  of  the  cell.  What- 
ever connects  the  poles  is  called  the  circuit. 

The  electric  current  is  always  assumed  to  Row  from 
the  passive  plate,  or  positive  pole,  through  the  wire  to 
the  active  plate,  or  negative  pole. 

307.  Bunseris  Cell.  —  When  the  above  voltaic  cell  is 
in  action,  bubbles  of  hydrogen  collect  upon  the  passive 
plate  so  as  nearly  to  cover  it.     This  prevents  contact  be- 
tween the    plate    and  the    liquid,  and  Fig.  173. 
interferes  with  the  chemical  action  of 
the  cell. 

In  Bunseris  cell  this  collection  of 
hydrogen  is  prevented  by  surround- 
ing the  passive  plate  with  strong  nitric 
acid,  which  takes  up  the  hydrogen. 
This  cell  (Figure  173)  consists  of  the 
following  parts :  a  large  earthen  or 
glass  cup  ;  a  piece  of  zinc  rolled  into 
a  cylinder  and  open  down  one  side  ;  a 
porous  porcelain  cup,  small  enough  to 
go  inside  the  zinc  cylinder;  and  a 
piece  of  coke  carbon,  small  enough  to  stand  inside  the 


200 


ELECTRICITY. 


Fig.  174. 


porcelain  cup.  The  larger  cup  is  filled  with  dilute 
sulphuric  acid,  and  the  smaller  cup  with  the  strongest 
nitric  acid.*  The  carbon  is  the  passive  plate. 

In  Grove's  cell,  a  strip  of  platinum  is  used  for  the  pas- 
sive plate,  instead  of  carbon.  In  other  respects  it  is 
essentially  the  same  as  Bunsen's. 

308.  DanielVs  Cell.  —  This  cell  is  shown  in  Figure 
174.     The  outer  vessel  is  of  copper,  and  serves  .as  the 

passive  plate.  Inside  this  is  a  vessel  of 
porous  earthen-ware,  containing  a  rod  of 
zinc.  The  space  between  the  copper  and 
the  porous  cup  is  filled  with  a  solution 
of  blue  vitriol,  which  is  kept  saturated  by 
crystals  of  the  salt  lying  on  a  perforated 
shelf.  The  porous  cup  is  filled  with  dilute 
sulphuric  acid.  The  porous  partition 
keeps  the  fluids  from  mingling,  but  does 
not  hinder  the  passage  of  the  current. 

The  blue  vitriol  in  contact  with  the  passive  plate  serves 

to  take  up  the  hydrogen. 

309.  The   Electric  Battery.  —  Several  cells  joined 
together  constitute  a  battery.     There  are  two  ways  in 
which  the  cells  may  be  joined  :   (i)  the  zinc  of  the  first 
cell  may  be  joined  to  the  carbon  of  the  second,  and  the 
zinc  of  the  second  to  the  carbon  of  the  third,  and  so 
on  throughout,   and  the  free  carbon  of  the  first  cell 
joined  to  the  free  zinc  of  the  last  by  a  wire,  as  in  Fig- 
Fig-  175- 


*  Instead  of  nitric  acid,  we  may  use  a  mixture  of  equal  parts 
of  strong  sulphuric  acid  and  a  concentrated  solution  of  bichro- 
mate of  potash. 


ELECTRICITY.  2OI 

ure  175  ;  or  (2)  the  zincs  may  all  be  joined  together, 
and  the  carbons  all  joined  together,  and  then  the  zincs 
and  the  carbons  joined  by  a  wire,  as  in  Figure  176. 

310.  Quantity  and  Intensity.  —  Arrange  a  battery  of 
three  cells  according  to  the  first  method,  and  connect  the 
poles  by  a  short,  thick  copper  wire,  which  passes  over  a 
needle,  and  observe  how  much  the  needle  is  turned  aside, 
or  dejlected.  Then  arrange  the  same  cells  according  to 
the  second  method,  and  connect  the  zinc  and  carbon  by 
the  same  wire ;  and  the  needle  will  be  dejlected  more 
than  in  the  former  case.  When  the  battery,  then,  is 
arranged  according  to  the  second  method,  the  current 
has  the  greater  power  to  dejlect  the  needle. 

Fig.  176. 


If  now  the  cells  be  again  arranged  in  the  Jirst  way, 
and  a  piece  vijine  steel  wire,  two  or  three  feet  long,  be 
put  into  the  circuit,  the  needle  will  be  dejlected  consid- 
erably less  than  when  the  circuit  is  completed  with  the 
short,  thick  copper  wire;  showing  that  the  current  is 
resisted  in  passing  through  thejine  wire.  If  the  same 
piece  of  fine  wire  be  put  into  the  circuit  when  the  battery 
is  arranged  according  to  the  second  method,  the  needle 
will  be  dejlected  considerably  less  than  before  ;  showing 
that  the  current  produced  by  \hejirst  form  of  battery  has 
the  greater  power  of  overcoming  resistance. 

The  power  of  the  current  to  dejlect  a  needle  is  called 
its  quantity,  and  its  power  to  overcome  resistance  in  the 
circuit  is  called  its  intensity,  or  its  tension. 

The  Jirst  form  of  the  battery  develops  electricity  of 
the  greatest  tension,  and  is  called  a  battery  of  tension,  or 


2O2 


ELECTRICITY. 


Fig.  177. 


intensity  battery;  while  the  second 
form  of  the  battery  develops  electricity 
in  the  greatest  quantity,  and  is  called 
a  battery  of  quantity,  or  quantity 
battery. 

When    electricity    in     considerable 
quantity  and  of   considerable  tension 
is  required,  the  two  methods  of  ar- 
ranging the   battery   are   combined, 
as  represented  in  Figure  177* 

C3ii.  Conductors  and  Non- Conductors. — If  a  piece 
>f  glass,  sealing-wax,  or  dry  wood  be  put  into  the  circuit, 
no  current  passes.  Substances  which  will  not  allow  the 
electric  force  to  pass  through  them  are  called  non-con- 
ductors; while  those  through  which  it  passes  freely  are 
called  conductors.  Metals  are  generally  good  conduc- 
tors. Copper  is  one  of  the  best  conductors,  and  is  gen- 
erally used  for  transmitting  the '  electric  current. 

When  the  circuit  is  composed  entirely  of  conductors, 
it  is  called  a  closed  circuit ;  when  there  is  a  non-conduc- 
tor in  any  part  of  the  circuit,  it  is  called  an  open  cir- 
cuit. 

312.  The  Rheotome  and  the  Rheotrope.  — An  instru- 
ment for  breaking  the  current  is  called  a  rheotome,  a 
name   derived   from   two  Greek   words,   and  signifying 
current-cutter.     An  instrument  for  changing  the  di- 
rection of  the  current  is  called  a  rheotrope;  that  is,  a 
current-turner.     These  two  instruments  are  often  com- 
bined in  one. 

313.  A   Magnetic  Needle   tends   to  place   itself  at 
right  angles  with  a  Wire  through  which   a    Current 
isjlowing.  —  If  the  current  be  made  to  flow  over  a  needle 
from  its  north  end  to  its  south  end,  the  north  pole  of  the 
needle  will  turn  to  the  left  hand  of  an  observer  who  is 
facing  that  pole.     If  it  be  made  to  pass  over  the  needle 


ELECTRICITY.  203 

from  its  south  pole  to  its  north,  its  north  pole  will  turn  to 
the  right.  If  it  be  made  to  flow  under  the  needle,  the 
north  pole  will  turn  in  just  the  opposite  direction.  We 
see,  then,  that  the  needle  always  tends  to  place  itself  at 
right  angles  to  a  wire  through  which  a  current  is 
flowing. 

314.  The  Rheoscope,  or  Galvanometer.  —  An  instru- 
nment   used  for  detecting  or  measuring  a  current   is 

called  a  rheoscope,  or  a  galvanometer.  The  first  name 
means  a  current-examiner;  the  second,  a  measurer  of 
galvanism.  Current  electricity  is  often  called  galvan- 
ism, from  its  discoverer,  Galvani. 

If  the  magnetic  needle  used  to  detect  the  electricity 
(306)  moves  over  a  graduated  arc,  it  will  be  found  to 
move  a  greater  number  of  degrees  when  the  current 
passes  entirely  round  it,  than  when  it  merely  passes 
over  it  or  under  it.  The  effect  is  the  same  as  if  two 
currents  of  equal  strength  were  passing  over  or  under 
the  needle,  and  both  in  the  same  direction.  Every  time, 
therefore,  that  the  wire  conducting  the  current  is  coiled 
round  the  needle,  the  effect  of  the  current  is  multiplied. 
A  current  which  is  too  weak  to  deflect  the  needle  by 
simply  passing  over  or  under  it,  may  be  made  to  deflect 
it  decidedly  by  coiling  the  conducting  wire  many  times 
round  the  needle. 

315.  The  Astatic  Needle.  —  When  a  single  needle  is 
deflected   by  the   current,  the   directive   action   of  the 
earth,  which  tends   to   make   the   needle   take  a  north 
and  south  position,  offers  a  resistance  to  the  deflection. 
In  order  to  neutralize  this  directive  action,  two  needles 
of  equal  magnetic  strength  are  fastened  together,  so 
that  the  north  pole  of  one  faces  the  south  pole  of  the 
other,  as  in  Figure  178.     Since  the  earth  will  pull  each 
end    of    such   a   compound    needle   towards    the    north 
and    towards   the    south    with    equal    strength,    it  will 


204  ELECTRICITY. 

Fig.  178.  have  no  tendency  to  point  north  and 

south.  This  needle  is  called  an 
astatic  needle  (from  a  Greek  word 
meaning  unsteady)  ;  that  is,  one 
having  no  directive  power.  An 
astatic  galvanometer  is  one  in 
which  the  needle  is  of  this  kind. 

316.  The  Resistance  of  Conductors.  —  It  has  been 
proved  that  the  resistances  of  'wires  of  the  same  ma- 
terial and  of  uniform  thickness  to  the  current  are  in 
the  direct  ratio  of  their  lengths,  and  in  the  inverse 
ratio  of  the  squares  of  their  diameters.  Thus  a  wire 
of  a  certain  length  offers  twice  the  resistance  of  its  half, 
thrice  that  of  its  third,  and  so  forth.  Again,  wires  of  the 
same  metal,  whose  diameters  are  in  the  ratio  of  i,  2,  3, 
etc.,  offer  resistances  which  are  to  each  other  as  i,  £,  £, 
etc.  Therefore,  the  longer  the  wire,  the  greater  the 
resistance;  the  thicker  the  wire, 'the  less  the  resistance. 
The  same  holds  true  of  liquids,  but  not  with  the  same 
exactness. 


SUMMARY. 

A  voltaic  pair  or  cell  consists  of  two  plates,  usually  of 
metal,  immersed  in  a  liquid  which  will  act  chemically 
upon  one  of  them.  The  plate  acted  upon  is  called  the 
active  plate,  the  other  the  passive  plate. 

The  two  plates  are  called  the  poles  of  the  cell ;  the 
passive  the  positive  pole,  and  the  active  the  negative 
pole.  That  which  is  employed  to  connect  the  poles  is 
called  the  circuit.  (306.) 

A  force  called  electricity  resides  in  a  wire  which  con- 
nects the  poles.  Since  this  force  seems  to  flow  through 
the  wire,  it  is  called  a  current.  We  always  consider  the 


ELECTRICITY.  2C>5 

current  as  starting  from  the  positive  pole,  and  passing  to 
the  negative  pole.     (306,311.) 

When  two  or  more  cells  are  connected,  the  apparatus 
is  called  a  battery. 

The  carbon  of  one  cell  may  be  connected  with  the  zinc 
of  the  next,  and  so  on  throughout ;  or  the  carbons  may 
all  be  connected,  and  also  all  the  zincs.  In  the  first  case, 
the  free  zinc  of  the  first  cell  and  the  free  carbon  of  the 
last  are  the  poles  of  the  battery  ;  in  the  second,  the  united 
carbons  constitute  one  pole,  and  the  united  zincs  the 
other.  (309.) 

The  power  of  the  current  to  turn  a  needle  is  called  its 
quantity;  its  power  to  overcome  resistance,  its  intensity. 

The  first  form  of  battery  gives  electricity  of  greater 
intensity  than  the  second  form ;  while  the  latter  gives 
electricity  of  greater  quantity  than  the  other.  (310.) 

Substances  which  allow  the  current  to  pass  readily 
, through  them  are  called  conductors;  those  which  will 
not  allow  it  to  pass  are  called  non-conductors.  A  closed 
circuit  is  made  up  entirely  of  conductors.  If  there  is  a 
non-conductor  in  the  circuit,  it  is  said  to  be  open.  (311.) 

An  instrument  for  breaking  the  current  is  called  a 
rheotome;  an  instrument  for  changing  the  direction  of 
the  current,  a  rheotrope.  (312.) 

A  magnetic  needle  tends  to  place  itself  at  right  angles 
to  a  wire  through  which  a  current  is  passing.  The  direc- 
tion in  which  the  needle  turns  depends  on  the  direction 
of  the  current,  and  upon  the  position  of  the  needle  with 
reference  to  the  wire.  (313.) 

An  instrument  for  indicating  and  measuring  the  cur- 
rent is  called  a  rheoscope,  or  galvanometer.  (314.) 

The  resistance  to  the  deflection  of  the  needle  caused 
by  the  magnetic  attraction  of  the  earth  is  neutralized  in 
the  astatic  needle,  which  is  therefore  more  sensitive  to  the 
action  of  the  current.  (315.) 


2O6  ELECTRICITY. 

The  effect  of  the  current  upon  a  needle  is  multiplied  by 
coiling  the  wire  round  the  needle.  (314.) 

The  resistances  oif  wires  of  the  same  material  and 
thickness  are  directly  as  their  lengths,  and  inversely  as 
the  squares  of  their  diameters.  (316.) 


ELECTRO-MAGNETISM. 

317.  The  Current  can  make  Iron  magnetic.  —  If  a 
part  of  the  wire  of  the  circuit  be  wound  into  a  coil, 
a  piece  of  soft  iron  placed  inside  this  coil  becomes 
strongly  magnetic  while  the  current  is  passing,  and 
is  called  an  electro-magnet.  The  coil  is  called  a  helix. 
When  the  current  passes  through  the  coil  in  the  direc- 
tion of  the  hands  of  a  watch,  the  end  at  which  it  enters 
will  be  a  south  pole,  as  in  Figure  1 79 ;  so  that,  by 

Fig.  179. 


reversing  the  current,  the  poles  of  the1  electro-magnet 

"will  be  reversed. 

When  the  current  is  broken,*  the  soft  iron   instantly 

loses  its  magnetism.     A  steel  rod  retains  its  magnetism 

after  the  current  is  broken.     If  the  wire  is  wound  around 
'  Fig.  180.  the  iron  in  several  layers,  the  strength 

of  the  magnet  is  greatly  increased. 

The  strongest  electro-magnets  are 
of  the  horseshoe  form.  They  far 
exceed  ordinary  magnets  in  power. 
Small  electro  -  magnets  have  been 
made  which  support  3,500  times 
their  own  weight,  and  large  ones 
which  hold  up  a  weight  of  2,500 

pounds.     These  magnets  are  much  stronger  when  pro- 


ELECTRICITY. 


207 


vided  with  a  keeper,  or  armature;  that  is,  a  piece  of 
soft  iron  to  connect  the  poles,  as  in  Figure  180. 

318.  The  Wire  through  which  a  Current  is  passing 
is  a  Magnet.  —  If  the   current  be   sent   through   a  coil 
such  as  is  shown  in  Figure  181,  and  the  end        Fig.  181. 
of  a  rod  of  soft  iron  be  brought  near  the 

opening  in  the  centre,  it  is  at  once  drawn 
into  the  coil.  Coils  have  been  made  which 
would  draw  up  a  weight  of  600  pounds. 

If  the  wire  joining  the  poles  of  a  battery 
is  brought  in  contact  with  fine  iron  filings, 
they  adhere  to  the  wire ;  showing  that  any 
'wire  through  which  the  current  is  flowing 
is  magnetic. 

319.  Electricity  as  a  Source  of  Mechan- 
ical Power.  —  All  the  electro-magnetic  ma- 
chines which  have  been  invented  for  doing 
work,  depend  on  the  property  of  an  electro- 
magnet to  acquire  or  to  lose  its  magnetism  when  the 
current  flows  or  is  interrupted;   or  to  reverse  its  poles 
when  the  direction  of  the  current  changes. 

Page's  rotating  machine  (Figure  182)  illustrates  one 
method  of  making  the  electric  force 
do  work.  It  consists  of  a  horseshoe 
magnet,  in  the  axis  of  which  is  an 
upright  shaft.  To  this  a  piece  of 
soft  iron  is  fixed,  with  its,  ends  fa- 
cing the  poles  of  the  magnet.  The 
soft  iron  is  surrounded  with  a  coil 
of  copper  wire,  so  that  it  is  an  elec- 
tro-magnet. The  ends  of  the  wires 
of  the  coil  are  fastened  to  two  metal- 
lic strips,  which  are  attached  to  the 
shaft.  The  current  comes  to  the 
coil  through  two  springs  which 


Fig.  182. 


208 


ELECTRICITY. 


press  against  these  strips,  and  which  act  as  a  rheotrope 
to  reverse  the  current  when  the  shaft  has  turned  half-way 
round. 

The  machine  is  so  arranged  that,  at  starting,  the  poles 
of  the  two  magnets  facing  each  other  are  of  the  same 
kind.  They  therefore  repel  each  other,  and  when  the 
shaft  is  once  started,  they  send  it  around  a  quarter  of 
the  way;  then  unlike  poles  begin  to  approach  each 
other,  and  their  attraction  causes  the  shaft  to  complete 
half  a  rotation.  The  current  then  changes  its  direction, 
the  poles  of  the  electro-magnet  are  reversed,  and  like 
poles  again  face  each  other  and  are  repelled.  The 
rotation  is  kept  up  by  the  self-acting  rheotrope.  The 
shaft  may  be  made  to  rotate  2,000  times  a  minute,  caus- 
ing 4,000  changes  of  polarity  in  that  brief  time. 

320.  Electric  Clocks.  —  The  electric  force  has  also 
been  used  to  regulate  the  movements  of  clocks,  called 
copying  clocks.  They  are  of -the  usual  construction, 
except  that  the  pendulum  balls  are  hollow  coils  of 
copper  wire,  which  become  mag- 
netic when  a  current  is  sent  through 
them.  In  Figure  183,  R  represents 
a  part  of  the  rod,  and  B  the  ball,  of 
such  a  pendulum.  Permanent  mag- 
nets, NS  and  S '  N,  are  fastened 
against  the  sides  of  the  clock-case 
opposite  the  ends  of  the  coil  B, 
with  like  poles  towards  the  coil. 
The  hollow  of  the  coil,  as  it  swings, 
can  pass  a  little  way  up  the  length  of  each  magnet.  If 
the  south  poles  of  the  magnets  are  turned  towards  the 
coil,  as  in  the  figure,  and  a  current  is  sent  through  the 
wire,  one  end  of  the  coil  becomes  a  north  pole,  which  is 
attracted  by  the  magnet  near  it,  and  the  other  end  a 
south  pole,  which  is  repelled  by  the  magnet  near  it. 


Fig.  183. 

•f- 


ELECTRICITY. 


209 


This  attraction  and  repulsion  both  tend  to  send  the 
coil  in  one  direction.  If,  now,  at  the  instant  that  B 
is  drawn  to  one  side,  the  direction  of  the  current  is 
changed,  the  poles  of  the  coil  are  reversed,  and  it  is 
carried  to  the  other  side.  The  pendulum  thus  vibrates 
every  time  the  current  is  reversed.  This  is  done  by 
means  of  a  regulating  clock.  Every  time  the  pendulum 
of  this  clock  vibrates,  the  current  is  reversed ;  so  that 
the  pendulums  of  all  the  copying  clocks  vibrate  exactly 
at  the  same  rate  as  the  pendulum  of  the  regulating 
clock. 

Figure  184  shows  one  of  the  ways  in  which  the  pendu- 
lum, A)  of  the  regulating  clock  can  change  the  direction 

Fig.  184. 


!1 

'-} 

1 

1 

! 

i 

i 

A 

i 

C 

B 

> 

/rf. 

! 

C 

D                             C 

S        r 

'N 

J      c 

3 

|    G    1 

E 

of  the  current.  The  spring  e  is  connected  with  the  neg- 
ative pole  of  the  battery  G,  and  the  spring  d  with  the 
positive  pole  of  the  battery  F.  The  other  poles  of  these 
batteries  are  connected  with  the  plates  m  and  n,  buried 
in  the  earth.  B  and  C  are  the  pendulums  of  the  copy- 
ing clocks.  When  the  regulating  pendulum  touches  the 
spring  </,  the  current  flows  through  the  wire  from  A  to 
B  and  C;  when  it  touches  the  spring  e,  the  current  flows 
first  through  the  earth  from  n  to  o,  and  then  through  the 
wire  from  C  to  A.  The  permanent  magnets  connected 
with  the  pendulums  B  and  C  do  not  appear  in  the 
figure. 


2IO  ELECTRICITY. 

321.  The  Electric  Telegraph.  —  An  instrument  for 
sending  signals  between  distant  stations  is  called  a  tele- 
graph.    The  word  means  writing  at  a  distance. 

Four  things  are  essential  in  every  electric  telegraph : 
(i)  a  battery  for  generating  electricity  ;  (2)  wires  to  con- 
duct the  electricity;  (3)  an  instrument  for  sending  the 
message ;  and  (4)  an  instrument  for  receiving  the  mes- 
sage. 

The  battery  used  is,  in  almost  all  cases,  a  voltaic  bat- 
tery. The  sending  instrument  is  merely  a  key  for  open- 
ing and  closing  the  circuit,  or  for  changing  the  direction 
of  the  current.  The  receiving  instrument,  in  the  needle 
telegraph,  is  a  magnetic  needle,  which  by  its  move- 
ments indicates  the  message  sent.  In  Bairfs  chemical 
telegraph,  an  iron  point  makes  blue  marks  on  paper 
by  the  action  of  the  current  upon  a  compound  (prus- 
siate  of  potash)  with  which  the  paper  has  been  moist- 
ened. 

322.  Morse's    Telegraph.  —  This   telegraph  'depends 
von  the  power  of  the  current  to  develop  magnetism  in 

soft  iron,  and  hence  is  called  the  electro-magnetic  tele- 
graph. 

The  essential  parts  of  the  receiving  instrument  are 
shown  in  Figure  185.  One  of  the  screw-cups  at  the 
right  is  connected  with  the  wire  from  the  distant  station, 
and  the  other  with  the  earth.  The  current  traverses  the 
coils  of  the  electro-magnet,  and  draws  down  the  keeper 
and  the  arm  of  the  lever  to  which  it  is  attached.  The 
other  end  of  the  lever  is  raised,  pressing  a  steel  point,  or 
style,  against  a  strip  of  paper,  which  is  unrolled  from 
the  bobbin  above,  and  moved  steadily  along  by  clock- 
work not  represented  in  the  figure.  When  the  current 
from  the  distant  station  is  broken,  the  shorter  arm  of  the 
lever  is  released  by  the  electro-magnet,  the  longer  arm 
falls  back  by  its  weight,  and  the  style  ceases  to  press 


ELECTRICITY.  211 

against  the  paper.  If  the  style  is  raised  for  a  moment 
only,  a  dot  is  made ;  if  for  a  longer  time,  a  dash.  The 
alphabet  used  is  made  up  by  the  combination  of  dots 
and  dashes. 

323.    The  Earth  may  serve  as  a  Telegraphic  Wire. 
—  One  wire  is  sufficient  to  connect  two  telegraph  sta- 

Fig.  185. 


tions,  if  its  terminations  be  formed  by  two  large  plates 
buried  so  deep  that  the  earth  about  them  never  gets 
dry.  The  earth  serves  the  purpose,  not  only  of  a  second 
wire,  but  of  one  so  thick  that  its  resistance  is  next  to 
nothing  (316). 

324.  The  Relay.  —  On  long  circuits  there  is  a  great 
loss  of  electricity  by  leakage  on  the  way,  so  that  a  cur- 
rent strong  at  starting  becomes  very  weak  before  it 
reaches  the  station  to  which  it  is  sent.  On  such  circuits 
it  is  usual  to  work  the  receiving  instrument  by  a  local 
current,  and  to  include  in  the  line  circuit  a  very  delicate 
instrument,  called  the  relay,  which  has  only  to  make  or 
break  the  local  circuit.  The  electro-magnet  E,  of  the 


212  ELECTRICITY. 

relay  (Figure  186)  is  included  in  the  line  circuit,  instead 
Fig.  186.  of    the    electro  -  magnet 

of  the  receiving  instru- 
ment. The  coil  is  long, 
and  of  very  fine  wire  ; 
and  a  very  faint  cur- 
rent is  sufficient  to  de- 
velop magnetism  in  the 
core.  The  keeper,  A, 
of  the  relay  is  attached 
to  a  lever,  ee1,  turning 
on  the  axis  a.  When  a  current  is  sent  through  the  coil, 
the  lever  is  drawn  down,  and  the  end  e?  rests  on  the 
screw  S.  When  there  is 'no  current,  the  spring  s  brings 
e1  back  against  the  insulated  screw  S'.  The  pillars  JV 
and  P  are  connected  with  the  poles  of  the  local  bat- 
tery. The  metal  spring  s  places  the  lever  e  e1  in  connec- 
tion with  P.  The  screw  61  and  the  end  e'  of  the  lever, 
then,  are  virtually  the  poles  of  this  battery.  When  these 
are  in  contact,  the  local  current  flows,  and  it  stops  when 
e'  is  brought  back  against  the  screw  S'.  The  receiving 
instrument  is  included  in  the  local  circuit.  When  a  cur- 
rent comes  from  the  sending  station,  the  keeper  A  is 
attracted,  e'  falls  on  S,  the  local  circuit  is  closed,  and  the 
receiving  instrument  begins  to  print.  When  the  current 
ceases,  e'  returns  to  S',  and  the  style  of  the  receiving  in- 
strument is  withdrawn  from  the  paper.  By  this  means, 
a  current  too  'weak  to  'work  the  receiving1  instrument 
can  complete  the  local  circuit  and  print  legibly. 

325.  The  Telegraphic  Fire-Alarm.  —  The  electric 
telegraph  is  now  extensively  used  for  indicating  the 
locality  of  fires  in  cities.  In  various  parts  of  the  city 
are  small  iron  boxes  called  signal-boxes.  They  are  all 
numbered,  and  connected  with  a  central  station  by  means 
of  wires.  By  turning  a  crank  which  is  found  inside  the 


ELECTRICITY. 


213 


signal-box,  the  circuit  is  opened  and  closed  in  such  a  way 
as  to  telegraph  to  the  central  station  the  number  of  the 
box.  When,  therefore,  a  fire  occurs  in  the  neighborhood 
of  any  box,  the  box  is  opened,  the  crank  turned,  and  the 
number  of  the  box  telegraphed  to  the  central  station. 
This  station  is  also  connected  by  wire  circuits  with  bells 
in  different  parts  of  the  city,  and  the  operator,  by  means 
of  the  electric  force,  rings  on  these  bells  the  number  of 
the  box,  so  that  the  firemen  know  at  once  the  neighbor- 
hood to  which  they  must  go. 

If  the  number  of  the  box  is  ten  or  less,  it  is  indicated 
by  a  corresponding  number  of  strokes  on  the  bell.  If 
above  ten,  the  digits  of  the  number  are  indicated  by 
striking  the  numbers  corresponding  to  them  with  a 
short  pause  between.  Thus  to  strike  the  number  25, 
two  blows  would  be  given,  and  then  after  a  pause  five 
more.  Numbers  containing  ciphers  and  those  made  up 
of  figures  repeated,  as  22,  33,  etc.,  are  not  used  for  the 
„  signal-boxes. 

SUMMARY. 

The  wire  through  which  a  current  flows  is  magnetic. 

This  magnetism  appears  much  stronger  when  the  wire 
is  bent  into  a  coil,  or  helix. 

When  a  piece  of  soft  iron  is  placed  inside  a  coil  and  a 
current  sent  through  the  wire,  it  becomes  magnetic.  A 
magnet  made  in  this  way  is  called  an  electro-magnet, 
X^v  and  is  much  stronger  than  an  ordinary  magnet. 

If  the  core  of  an  electro-magnet  is  of  soft  iron,  the 
magnetism  can  be  destroyed  by  breaking  the  current, 
and  the  poles  can  be  reversed  by  changing  the  direction 
of  the  current.  If  the  core  is  of  steel,  it  retains  its  mag- 
netism permanently.  (317?  S1^') 

Page's  rotating  apparatus  illustrates  one  of  the  ways 


214  ELECTRICITY. 

in  which  the  electric  force  may  be  made  to  do  mechanical 
work.  (319.) 

The  electric  force  has  been  used  for  regulating  the 
motion  of  clocks.  (320.) 

Four  things  are  essential  in  an  electric  telegraph ;  a 
battery,  a  conducting  wire,  a  sending  instrument,  and 
a  receiving  instrument.  (321.) 

Morse's  telegraph  depends  on  the  power  of  the  cur- 
rent to  develop  magnetism.  (322.) 

The  main  battery  is  made  to  work  a  relay  magnet,  and 
a  local  battery  to  work  the  receiving  instrument.  By 
means  of  the  relay  magnet  the  operator  can  open  and 
close  the  circuit  of  the  local  battery  at  a  distance.  (324.) 

The  electric  fire-alarm  is  another  form  of  the  electro- 
magnetic telegraph.  (325.) 

ELECTROLYSIS. 

326.  A  Compound  may  be  decomposed  by  the  Cur- 
rent.—  A  compound  substance,  as  water,  may  be  decom- 
posed^ or  separated  into  its  elements,  by  the  electric 
current.  This  decomposition  by  electricity  is  called 
electrolysis.  The  literal  meaning  of  the  word  is  loosen- 
ing by  electricity.  The  substance  decomposed  is  called 
the  electrolyte.  The  metallic  conductors  through  which 
the  current  passes  into  and  out  of  the  electrolyte  are 
called  electrodes  (roads  of  electricity).  That  through 
which  the  electricity  passes  in  is  termed  the  anode  (road 
up)  ;  and  that  through  which  it  passes  out,  the  cathode 
(road  down).  'The  electrolyte  is  always  separated  into 
two  parts,  one  of  which  appears  at  the  anode  and  the 
other  at  the  cathode. 

Water  is  a  compound  of  two  gases,  oxygen  and  hydro- 
gen, into  which  it  can  be  separated  by  electrolysis.  The 


ELECTRICITY.  21$ 

oxygen  appears  at  the  anode,  and  the  hydrogen  at  the 
cathode. 

Every  compound  liquid  which  is  a  conductor  of  elec- 
tricity may  be  decomposed  by  the  current.  Solid  com- 
pounds are  not  thus  decomposed. 

327.  The  Electrolysis  of  Blue  Vitriol.  —  If  two  elec- 
trodes of  platinum  be  put  into  a  solution  of  blue  vitriol 
(Figure  187),  bubbles  of  gas  rise  from  the  anode.     This 
gas  is  found  to  be  oxygen,  which  is  one  of  the  elements 
in  the  blue  vitriol.    On  removing  the  cathode        F;g  l8? 
from  the   solution,  it  is  found  to  be  coated 

with  copper,  which  is  another  element  in  the 

blue  vitriol.     If  one  of  the  electrodes  be  of 

platinum  and  the  other  of  copper,  and  the 

platinum  be  made  the  anode,  the  same  results 

are  obtained.      If,   however,   the  copper  be 

made  the  anode,  the  cathode  is  still  coated 

with  copper,  but  no  gas  escapes  from  the  anode.     In  this 

case,  the  anode  is  gradually  dissolved  in  the  liquid,  and 

transferred  to  the  cathode. 

When  any  compound  containing  a  metal  is  decom- 
posed by  electricity,  the  metal  always  appears  at  the 
cathode;  and  if  the  anode  be  of  the  same  metal,  it  is 
gradually  transferred  to  the  cathode. 

328.  Electrotyping.  —  When  the  solution  of  blue  vit- 
riol is  decomposed  slowly,  the  copper  is  deposited  on  the 
cathode  in  a  tenacious  mass,  which,  when  stripped  off, 
presents  a  perfect  reverse  image  of  the  face  of  the  cath- 
ode.   If  this  reverse  image  be  now  made  the  cathode,  and 
another  sheet  of  copper  be  deposited  upon  it,  an  exact 
copy  of  the  original  electrode  is  obtained.     Any  con- 
ducting substance  may  be  made  a  cathode  by  simply  con- 
necting it  with  the  negative  pole  of  the  battery.     Hence 
coins,  medals,  and  engraved  plates  may  be  copied  with 
perfect  accuracy,  and  with  but  slight  trouble  and  expense. 


2l6  ELECTRICITY. 

This  process  of  copying  by  means  of  electricity  is 
called  electrotyping. 

The  face  of  a  medal  may  be  copied  by  making  it  the 
cathode  and  depositing  a  sheet  of  copper  upon  it,  and 
then  depositing  another  sheet  of  copper  upon  this  sheet 
after  it  has  been  separated  from  the  medal.  In  practice, 
however,  a  mould  of  the  thing  to  be  copied  is  first  taken 
in  some  soft  substance,  such  as  plaster,  gutta-percha,  or 
wax,  and  this  mould  is  made  the  cathode.  If  the  mould 
is  made  of  non-conducting  material,  as  is  usually  the  case, 
its  surface  must  be  covered  with  some  conducting  sub- 
stance, as  powdered  graphite. 

One  of  the  chief  uses  of  electrotyping  is  in  copying 
printer's  type  after  it  has  been  set  up,  and  in  copy- 
ing 'wood  engravings.  This  book  is  printed  from  such 
copies,  or  electrotype  plates.  An  impression  is  taken 
of  the  type  or  the  engraving  in  wax,  which  is  then 
brushed  over  with  powdered  graphite,  and  made  the 
cathode ;  the  electrolyte  is  blue  vitriol,  and  the  anode 
a  piece  of  copper. 

329.  Electro-plating.  —  This  is  the  art  of  coating 
the  baser  metals  'with  silver  by  the  electric  current. 
Articles  to  be  electro-plated  are  generally  made  of  brass, 
bronze,  copper,  or  nickel  silver,  this  last  being  the  best 
material. 

The  bath  is  a  large  trough  of  earthen-ware  or  other 
non-conducting  substance.  It  contains  a  weak  solution 
of  argentic  cyanide  (cyanide  of  silver)  and  potassic 
cyanide  (cyanide  of  potassium).  A  plate  of  silver  forms 
the  anode ;  and  the  articles  to  be  plated,  hung  by  wires 
to  a  metal  rod  lying  across  the  trough,  constitute  the 
cathode.  When  the  former  is  connected  with  the  positive 
pole  of  a  battery,  and  the  latter  with  the  negative  pole, 
the  silver  of  the  cyanide  begins  to  deposit  itself  on  the 
suspended  articles;  while  the  silver  anode  is  dissolved 


ELECTRICITY. 

(327),  furnishing  a  fresh  supply  of  cyanide.  The  thick- 
ness of  the  plating  depends  on  the  length  of  time  the 
articles  are  immersed. 

330.  Electro-gilding.  —  This  process  is  essentially  the 
same  as  electro-plating,  except  that  the  articles  are  coated 
with  gold  instead  of  silver.    The  electrolyte  is  some  com- 
pound of  gold,  and  the  anode  is  a  lump  of  gold. 

331.  Electro-metallurgy.  —  The  art  of  depositing,  by 
electro-chemical  action,   a  metal  on  any  surface  pre- 
pared to  receive  it,  is  called  electro-metallurgy.     There 
are  two  kinds  of  electro-metallurgy,  one  of  which  is  illus- 
trated by  electrotyping,  and  the  other  by  electro-plating. 
The  former  includes  all  those  cases  in  which  the  coating 
of  metal  merely  adheres  to  the   surface  on  which  it  is 
deposited,  and  is  afterwards  stripped  off;  and  the  latter, 
all   cases  in  which  the  two  metals  combine  with  each 
other,  and  the  coating  remains  permanently  fixed.    Gold, 
platinum,  silver,  copper,  zinc,  tin,  lead,  cobalt,  and  nickel 
can  be  deposited  by  electrolysis. 


SUMMARY. 

When  any  compound  liquid  which  is  a  conductor  of 
electricity  forms  a  part  of  the  circuit,  it  is  decomposed. 
This  decomposition  by  electricity  is  called  electrolysis. 

When  the  electrolyte  contains  a  metal,  this  always 
appears  at  the  cathode ;  and  if  the  anode  is  of  the 
same  metal,  it  is  gradually  dissolved  and  deposited  on 
the  cathode.  Advantage  is  taken  of  this  fact  in  electro- 
typing,  electro-plating,  and  electro-gilding.  (326-330.) 

In  electrotyping,  the  metal  deposited  on  the  cathode 
merely  adheres  to  it,  and  is  afterwards  removed ;  in 
electro-plating  and  electro-gilding,  it  permanently  com- 
bines with  the  metal  on  which  it  is  deposited.  (331.) 


2l8  ELECTRICITY. 


POWER  OF   THE   CURRENT   TO   DEVELOP 
HEAT  AND    LIGHT. 

332.  Heat  is  developed  by  the  Current.  —  When  a 
current  passes  through  fine  wire,  an  intense  heat  is  pro- 
duced, sufficient  in  some  cases  to  bring  it  to  a  white  heat, 
and  even  to  fuse  platinum  wire.     If  the  wire  be  kept  the 
same,  or  of  the  same  resistance,  the  heat  is  in  propor- 
tion   to   the   square   of  the   strength   of  the   current. 
Thus,  if  a  current  of  a  certain   strength  raise  the  tem- 
perature i°  in  a  minute,  a  current  of  twice  the  strength 
will  raise  it  4°  in  a  minute. 

Again,  if  the  strength  of  the  current  be  kept  the  same, 
and  wires  of  different  resistance  be  tried,  the  heat 
developed  is  in  proportion  to  the  resistance  of  the 
wire.  Thus,  if  with  a  certain  wire  the  temperature 
be  raised  i°  per  minute,  it  will  be  raised  2°  per  minute 
with  a  wire  of  double  the  resistance. 

Hence  the  heat  developed  in  a  conducting  wire  by  an 
electric  current  is  proportional  to  the  squares  of  the 
strengths  of  the  current,  and  to  the  resistance  offered 
by  the  wire. 

The  power  of  the  current  to  ignite  fine  wires  of  com- 
paratively bad  conductors,  such  as  steel  and  platinum,  is 
used  to  explode  gunpowder  at  a  distance,  in  blasting  and 
mining.  The  current  is  transmitted  to  the  point  where 
the  explosion  is  to  take  place  by  good  conducting  wires, 
the  ends  of  which  are  connected  in  the  gunpowder  by 
fine  steel  wire.  When  the  current  is  sent  through  the 
wires,  the  fine  steel  wire  burns  up  and  explodes  the  gun- 
powder. 

333.  The  Electric  Light. —  When   the  poles   of  a 
powerful   battery  are   made    to    touch,  and  then    are 
separated  a  little,  the  current  forces  its  way  through 


ELECTRICITY.  219 

the  intervening  air,  ^producing  intense  light  and  heat. 
The  heat  is  sufficient  to  melt  the  most  refractory  metals, 
and  therefore  some  very  infusible  conductor  must  be  used 
for  the  poles.  The  best,  both  for  conducting  power  and 
durability,  is  the  coke  carbon  formed  in  the  distillation  of 
coal-gas. 

When  points  made  of  this  carbon  are  used  as  the 
poles,  and  are  separated  a  little,  while  a  strong  current 
is  passing  through  them,  a  light  appears  between  them 
rivalling  that  of  the  sun  in  purity  and  splendor.  This 
light  arises  chiefly  from  the  intense  whiteness  of  the  tips 
of  the  carbon  points,  and  partially  from  an  arch  of  flame 
extending  from  one  to  the  other. 

The  heat  of  this  arch  of  flame,  or  voltaic  arc,  as  it  is 
called,  is  the  most  intense  that  can  be  produced.  Plati- 
num melts  in  it  like  wax  in  the  flame  of  a  candle. 
Quartz,  the  sapphire,  magnesia,  lime,  and  even  the 
diamond,  are  readily  fused  by  it. 


SUMMARY. 

When  the  current  passes  through  a  conductor,  heat  is 
developed.  The  heat  is  proportional  to  the  square  of  the 
strength  of  the  current,  and  to  the  resistance  offered  by 
the  conductor. 

By  introducing  a  poor  conductor  into  any  part  of  the 
circuit,  heat  may  be  developed  at  that  point.  Advantage 
is  taken  of  this  fact  in  exploding  gunpowder  at  a  dis- 
tance, and  in  producing  the  electric  light.  (332,  333.) 

MAGNETO-ELECTRICITY. 

334.  An  Electric  Current  may  be  induced  by  a 
Magnet. — Attach  the  lifting-coil  to  the  galvanometer, 
place  the  rod  within  the  coil,  and  bring  it  quickly  in 


220 


ELECTRICITY. 


contact  with  the  pole  of  an  excited  electro-magnet. 
Magnetism  is  developed  in  the  rod,  and  the  galvanom- 
eter shows  a  current  in  the  wire  of  the  coil.  The 
needle  soon  returns  to  its  former  position.  Now  quickly 
detach  the  rod  and  coil  from  the  magnet.  The  rod  loses 
its  magnetism,  and  the  galvanometer  shows  a  current  in 
the  coil,  but  its  direction  is  the  opposite  of  that  of  the 
former  current. 

Electricity  thus  originated  by  a  magnet  is  said  to  be 
induced  by  it,  and  is  called  magneto-electricity. 

335.  Magneto-Electric  Machines.  —  An  instrument 
for  developing  magneto-electricity  is  called  a  magneto- 
electric  machine.    In  ordinary  machines  of  this  kind,  the 
electricity  is  induced  by  an  electro-magnet,  whose  mag- 
netism is  alternately  developed  and  destroyed  by  means 
either  of  a  permanent  magnet  or  of  an  electric  current. 

336.  Induction  Coils.  —  When  the  magnetism  is  de- 
veloped and  destroyed  by  means  of  a  current,  the  soft 
iron   must  be  placed  inside  a  coil   through  which  the 
current  is  sent.     This  is  called  the  primary  coil,  and 
must  be  placed  inside  another  coil,  called  the  secondary 

,.,.  coiL  which  serves  as  a 

r  ig.  loo. 

conductor  of  the  in- 
duced electricity .  Such 
a  magneto-electric  ma- 
chine is  called  an  in- 
duction coil.  In  the 
one  shown  in  Figure 
1 88,  the  primary  coil 
is  of  coarse  wire  wound 
with  wool,  and  is  at- 
tached to  the  wooden 
base  of  the  instrument. 
The  secondary  coil  is 
of  finer  silk-wound  wire,  much  longer  than  the  primary 


ELECTRICITY.  221 

wire.  Within  the  primary  coil  is  a  bundle  of  iron  wires, 
which  are  sufficiently  insulated  by  the  rust  that  gathers  on 
them.  The  developing  of  magnetism  in  these  wires  is 
the  chief  aim  of  the  primary  coil,  and,  as  this  requires  a 
strong  current,  coarse  wire  is  used  in  that  coil.  In  the 
secondary  coil,  the  aim  is  to  increase  the  tension  of 
the  induced  current,  and  fine  wire  is  used,  so  that  as 
many  turns  as  possible  may  be  brought  within  the  in- 
fluence of  the  primary  coil  and  its  core ;  for  it  is  found 
that  the  tension  of  the  induced  current  is  proportional 
to  the  strength  of  the  primary  current,  and  to  the 
square  of  the  resistance  in  the  secondary  coil. 

In  order  to  obtain  the  greatest  effect  from  the  secondary 
coil,  it  is  necessary  to  have  some  means  of  rapidly  com- 
pleting and  breaking  the  primary  current.  This  is 
done  either  by  means  of  the  rasp  seen  behind  the  coils, 
or  by  the  self-acting  rheotome  at  the  left  hand. 

337.  Extra  Current. — When  the  wires  of  a  small 
battery  are  short,  only  a  small  spark  is  obtained  on  sepa- 
rating them.  When  they  are  long,  and  especially  when, 
one  is  wound  into  a  coil,  a  much  larger  spark  is  obtained. 
This  is  due  to  an  extra  current  produced  by  the  induc- 
tive action  of  one  part  of  the  wire  upon  another,  as  the 
parts  successively  lose  their  magnetism.  The  longer  the 
circuit,  the  stronger  the  extra  current. 


SUMMARY. 

• 

Electricity  can  be  developed  by  magnetism,  and  is 
then  called  magneto-electricity.  In  all  ordinary  mag- 
neto-electric machines  the  electricity  is  induced  by  an 
electro-magnet,  which  is  excited  either  by  means  of  a  per- 
manent magnet,  or  of  the  electric  current.  In  the  latter 
case,  the  machine  is  usually  called  an  induction  coil. 


222 


ELECTRICITY. 


THERMO-ELECTRICITY. 


58.  Electricity  may  be  developed  by  Heat.  —  When 
the  point  of  junction  of  any  two  metals  is  heated,  a  cur- 
Fig.  189.      rent  is  always  produced.     When  a  bar  of  anti- 
mony, A,  is  soldered  to  a  bar  of  bismuth,  B 
(see  Figure  189),  and  their  free  ends  are  con- 
nected   with    a    galvanometer,    6r,    a    current 
passes  from  the  bismuth  to  the  antimony  when 
the  junction  is  heated.     When  S  is  cooled  by 
applying  ice,  or   otherwise,  a   current   in  the 
opposite  direction  is  produced.     Such  a  com- 
bination of  metals  is  called  a  thermo-electric 
pair.    Electricity  thus  developed  is  called  ther- 
mo-electricity (heat  electricity). 
Farmer's  alloy  (of  zinc  and  antimony)  forms  a  much 
more  powerful  pair  with  bismuth  than  antimony  does. 
•    339-    The  Thermopile.  —  One  bismuth-antimony  pair 
has  very  little  power.     To  obtain  a  stronger  current,  sev- 


Fig. 190. 


eral  pairs  are  united,  as  shown  in  Figure 
190.  The  heat  in  this  case  must  be 
applied  only  to  one  row  of  soldered 
faces.  The  strength  of  the  current  de- 
pends on  the  difference  of  temperature 
of  the  two  sides  ;  and  to  increase  it  to 
the  utmost,  one  series  must  be  kept  in 
ice  or  in  a  freezing-mixture,  whilst  the 
other  is  exposed  to  an  intense  heat.  As 
in  the  galvanic  battery,  the  electric  force 
is  proportionate  to  the  number  of  pairs. 

Figure  191  represents  a  common  form  of  thermo-elec- 
tric battery,  or  thermopile.  It  consists  of  thirty  pairs, 
arranged  as  in  Figure  190.  The  binding-screws  are 


3? 


ELECTRICITY.  223 

connected  with  the  bars  at  the  ends  of  the  series.  The 
bars  are  separated  by  a  non-conducting  substance,  as 
gypsum,  and  the  frame  is  made  of:  Fig  IQI 

non-conducting  material. 

The  thermopile,  in  connection 
with  a  sensitive  galvanometer, 
forms  the  most  delicate  of  differ- 
ential thermometers  (291).  So 
long  as  the  opposite  faces  are  ex- 
posed to  the  same  temperature,  no 
current  is  produced ;  but  if  the  temperature  of  one 
side  becomes  higher  than  that  of  the  other,  a  current 
is  at  once  indicated.  If  the  hand,  for  instance,  be 
brought  near  one  side,  the  needle  shows  a  current ;  or 
if  a  piece  of  ice  be  held  near,  a  current  is  also  shown, 
but  moving  in  the  opposite  direction. 


SUMMARY. 

Heat  has  power  to  develop  electricity  in  a  combination 
of  different  metals.  Electricity  thus  generated  is  called 
thermo-electricity.  (338.) 

The  thermopile  is  a  very  .sensitive  differential  ther- 
mometer, since  a  current  is  developed  by  the  slightest 
difference  of  temperature  between  the  two  faces.  (339.) 


FRICTIONAL  ELECTRICITY. 

340.  Electricity  may  be  developed  by  friction. — 
When  a  cat's  back  is  stroked  on  a  cold,  dry  day,  in  a 
darkened  room,  sparks  are  obtained  which  indicate  the 
development  of  electricity.  If  a  well-dried  rod  of  glass 
or  gutta-percha  be  rubbed  with  a  piece  of  silk  or  flannel, 


224  ELECTRICITY. 

similar  sparks  appear.  Electricity  thus  developed  by 
friction  is  called  frictional  electricity.  When  any  two 
dissimilar  bodies  are  rubbed  together,  electricity  is  de- 
veloped ;  but  when  the  substances  are  conductors,  the 
electricity  passes  off  silently  through  the  hands  and 
body.  In  order  to  detect  it,  the  substances  rubbed  to- 
gether must  be  held  by  insulating  handles  ;  that  is,  non- 
conducting handles. 

341.  The  Electrical  Machine.  —  An  apparatus  for 
generating  frictional  electricity  is  called  an  electrical 
machine.  The  one  shown  in  Figure  192  consists  of  a 

Fig.  192. 

X0  ' 


thick  plate  of  glass  turned  by  a  crank.  At  one  end  there 
is  a  glass  standard  surmounted  by  a  brass  ball.  From 
this  standard  project  two  brass  strips  in  the  form  of  a 
clamp,  which  hold  the  rubbers  against  the  glass  plate. 
These  'rubbers  are  pieces  of  wash-leather  or  woollen 
cloth,  covered  with  an  amalgam  of  mercury,  lead,  and 
tin.  At  the  opposite  end,  on  a  glass  support,  is  a  long 
cylinder  of  brass  with  rounded  ends,  called  the  prime  or 
positive  conductor.  The  brass  ball  connected  with  the 


ELECTRICITY.  225 

rubber  is  the  negative  conductor.     The  plate  and  con- 
ductors of  the  machine  must  be  well  insulated. 

342.  Quantity  and  Intensity  of  Frictional  Elec- 
tricity. —  With  a  medium-sized   machine  of  this  kind, 
sparks  are  readily  obtained  two  inches  long  by  bringing 
a  conducting  substance  near  the  ball  of  the  prime  con- 
ductor.    Very  large  machines  will  give  a  spark  two  feet 
in  length.     Frictional  electricity,  then,  must  have  great 
intensity,  in  order  to  traverse  so  great  a  distance  of  a  non- 
conducting substance  like  the  air.     Its  quantity,  on  the 
other  hand,  is  next  to  nothing.     This  is  shown  by  con- 
necting the  positive  conductor  with  one  end  of  the  wire 
of  a  moderately  delicate  galvanometer,  and  the  negative 
conductor  with  the  other  end,  and  working  the  machine. 
The  needle  will  be  turned  aside  scarcely  at  all.      The 
great  tension  and  the  small  quantity  of  frictional  elec- 
tricity place  it  in  striking  contrast  with  voltaic  electricity. 

The  positive  conductor  of  an  electrical  machine  an- 
swers to  the  positive  pole  of  a  galvanic  battery,  and  the 
negative  conductor  to  the  negative  pole,  and  the  friction 
on  the  plate  to  the  chemical  action  in  the  cells.  With 
the  galvanic  battery  an  enormous  quantity  of  electric- 
ity is  obtained  of  slight  tension;  with  the  electrical 
machine,  a  small  quantity,  of  enormous  tension^" 

343.  The  Electroscope.  —  If  a  pith  ball  hung  by  a  silk 
thread  from  a  glass  rod  be  brought  near  the  ball  of  a 
prime  conductor,  it  is  at  first  attracted  and       Fig.  I93. 
then    repelled.      This  power   of  attracting 

light  bodies  is  a  marked  feature  of  frictional 
electricity.  //  furnishes  the  most  ready 
means  of  detecting  the  presence  of  this 
electricity,  as  the  needle  furnishes  the  most 
ready  means  of  detecting  voltaic  electricity. 
An  instrument  for  the  detection  of  fric- 
tional electricity  is  called  an  electroscope. 

'5 


226  ELECTRICITY. 

The  pith-ball  electroscope  (Figure  193)  consists  of  a 
brass  conducting-rod  supporting  a  graduated  semicircle, 
Fig.  194.       in   the   centre  of  which  is  a  movable  index 
made  of  very  light  wood,  with  a  pith  ball  at 
the  end.     When  it  is  attached  to  the  prime 
conductor  of  the  machine,  the  pith  ball  is  re- 
pelled as  soon  as  the  plate  is  turned. 

The  gold-leaf  electroscope  (Figure  194)  is 
more  sensitive.  It  consists  of  a  hollow  glass 
ball,  through  the  cap  of  which  passes  a  brass 
rod  having  a  brass  ball  at  its  upper  end  and 
two  narrow  strips  of  gold-leaf  hung  from  its  lower  end. 
If  the  brass  ball  be  brought  near  a  body  charged  with 
electricity,  the  strips  of  gold-leaf  repel  each  other,  as  in 
the  figure. 

344.  The  Electrical  Forces  on  the  Positive  and  Neg- 
ative Conductors  act  in  Opposite  Directions.  —  Insu- 
late both  conductors,  and  charge  them  with  electricity. 
Bring  a  pith  ball  suspended  by  a  silk  thread  in  contact 
with   the   positive   conductor,   and   it  will    be   repelled. 
Take  it  now  to  the  negative  conductor,  and  it  will  be 
strongly  attracted.     A  ball  which  is  repelled  by  the  force 
on  one  conductor  is  attracted  by  the  force  on  the  other ; 
in  other  words,  the  two  forces  act  in  opposite  directions. 

345.  JBoth  Electrical  Forces  are  always  developed 
together.  —  It  is  impossible  to  develop  one  of  these  forces 
without  at  the  same  time  developing  both.     One  force 
always  appears  upon  one  of  the  substances  rubbed  to- 
gether,  and   the  other  force  always   appears   upon  the 
other.      The  force  that  acts  in  the  same  way  as  that 
upon  the  prime  conductor  of  the  machine  is  called  posi- 
tive electricity,  and  the  opposite  force  is  called  negative 
electricity.     In  order  that  both  the  forces  should  be  de- 
tected,  both  the    substances  rubbed  together   must  be 
insulated. 


ELECTRICITY.  227 

346.  Induction.  —  If  an  insulated  copper  ball  be  con- 
nected   with    the    prime     conductor 

when  charged,  and  a  small  insulated 
conductor  be  placed  near  it  (Figure 
195),  opposite  electrical  forces  will 
be  developed  upon  the  ends  of  the 
insulated  conductor.  On  the  end 
next  the  ball,  negative  force  will  be  found ;  on  the  end 
farthest  from  the  ball,  positive  force.  This  action  of  a 
charged  body  upon  a  body  near  it  is  called  induction. 
When  the  two  opposite  forces  exist  on  a  conductor,  it 
is  said  to  be  polarized;  'when  only  one  force  exists  on 
it,  to  be  charged;  and  'when  no  force  exists  on  it,  to  be 
neutral.  When  a  force  which  has  been  developed  on 
an  insulated  conductor  passes  off,  it  is  said  to  be  dis- 
charged. 

347.  The  Charge  on  a  Solid  Insulated  Conductor  is 
always  on  the  Surface.  —  To  an  insulated  copper. ball 
are  carefully  fitted  two  hemispherical  metallic  caps  pro- 
vided with    insulating   handles.      The   caps   are   placed 
upon   the   ball,  and   the  whole    apparatus    is    charged. 
The    caps    are    then   removed,    and    are  found    to    be 
charged,  while  not  the  slightest  trace  of  a  charge  is 
found  on  the  ball. 

When  a  spherical  conductor  is  charged  and  placed  in 
the  centre  of  a  room,  the  charge  is  distributed  uniformly 
over  its  surface;  if  the  conductor  be  oblong,  the  charge 
accumulates  at  the  ends. 

348.  The  Leyden^Jar. —  The  Leyden  jar  is  a  wide- 
mouthed  jar  of  thin  glass,  coated  with  tinfoil  on  both 
sides  to  within  an  inch  or  two  of  the  top,  and  closed 
with  a  stopper  of  cork  or  dry  wood,  through  which  a 
brass  rod  passes,  terminating  outside  in  a  ball,  and  con- 
nected inside  with  the  tinfoil  coating.    The  jar  is  charged 
by  connecting  its  outer  coating  with  one  conductor  of  an 


228  ELECTRICITY. 

electrical  machine  in  action,  and  the  inner  coating  with 
the  other.  Each  surface  of  the  glass  becomes  charged 
with  the  same  electric  force  as  the  conductor  with  which 
it  is  connected.  The  coatings  serve  merely  to  conduct 
the  electricity  over  the  surface  of  the  glass  in  charging 
and  discharging  the  jar. 

The  jar  may  be  discharged  by  means  of  the  discharger, 
which  consists  of  two  bent  brass  arms  connected  by  a 
movable  joint  and  having  brass  balls  at  their  ends.  It  is 
fastened  at  the  joint  to  a  glass  handle.  To  discharge  the 
jar,  hold  the  discharger  by  the  glass  handle,  and  bring 
one  ball  in  contact  with  the  outer  coating  and  the  other 
ball  near  the  knob  connected  with  the  inner  coating. 

349.  The  Leyden  Battery.  —  The  amount  of  charge 
which  a  Leyden  jar  can  receive,  othe^  things  being  equal, 
evidently  increases  with  the  size  of  the  coatings.     The 
area  of  the  coatings  can  be  most  conveniently  increased 
by  connecting  together  several  jars  as  a  Leyden  battery. 
Like  the  cells  of  the  voltaic  battery  (309),  the  jars  can  be 
connected  in  two  ways :   ( i )  the   outer  coating  of  one 
may  be  connected  with  the  inner  coating  of  the  next, 
and  so  on  throughout  the  series ;  or  (2)  the  outer  coat- 
ings may  all  be  connected  together,  and  also  the  inner 
coatings.     In  the  first  case,  the  battery  is  discharged  by 
bringing  the  inner  coating  of  the  first  jar  in  contact  with 
the  outer  coating  of  the  last ;    in  the    second  case,  by 
bringing  the  connected  outer  coatings  in  contact  with  the 
connected  inner  coatings.     Like  the  voltaic  battery,  when 
the  Leyden  battery  is  arranged  in  the  Jirst  way,  it  gives 
electricity  of  the  greatest  intensity;  and,  in  the  second 
way,  electricity  of  the  greatest  quantity. 

350.  The   Effect  of  Points  on  a  Conductor.  —  It  is 
impossible  to  charge  a  conductor  when   a   sharp   point 
projects  from  it,  or  is  held  near  it.      The  point  conveys 
away  the  electric  force  silently.     If  the  hand  be  held  in 


ELECTRICITY.  229 

front  of  the  point  when  the  electricity  is  developed,  a  cur- 
rent of  air  is  distinctly  felt  setting  off  from  the  point.  If 
a  lighted  taper  be  held  near  the  point,  the  flame  is  blown 
away  from  it.  The  electric  force  is  carried  off  by  the 
molecules  of  air  which  form  the  current,  and  hence  it  is 
called  convective  discharge*  Since  in  a  darkened  room 
a  star  of  light  is  seen  upon  a  point  held  near  an  elec- 
trical machine  in  action,  this  silent  discharge  is  also 
called  glow  discharge. 

The  charge  rises  so  high  at  the  point  that  the  molecules 
of  air  just  about  it  are  strongly  polarized.  They  then 
act  like  little  pith  balls,  being  first  drawn  to  the  point 
and  then  driven  from  it,  thus  producing  the  current  of  air. 

351.  The  Electric  Wheel.  —  As  each  molecule  is  re- 
pelled from  the  point,  it  also  repels  the  point  itself, 
which,  if  free  to  move,  will  be  set  in  motion.  Fig.I96. 
This  is  shown  by  the  electric  wheel  (Figure 
196),  which  consists  of  a  number  of  points  all 
bent  round  in  the  same  direction.  The  wheel 
is  poised  so  as  to  turn  easily,  and  when  con- 
nected with  the  prime  conductor  of  the  machine 
in  action,  it  rotates  rapidly,  each  point  moving  back- 
wards. 


SUMMARY. 

When  unlike  substances  are  rubbed  together,  fric- 
tional  electricity  is  developed.  (340.) 

Frictional  electricity  has  slight  quantity,  but  enormous 
tension ;  while  voltaic  electricity  has  slight  tension,  but 
enormous  quantity.  (342.) 

Two  opposite  electrical  forces  are  developed  on  the 
two  conductors  of  the  electrical  machine ;  and  one 
cannot  be  developed  without  the  other.  (344,  345-) 

A  body  is  polarized  when  it  has  opposite  electrical 


230  ELECTRICITY. 

forces  developed  on  opposite  parts ;  it  is  charged  when 
it  has  only  one  electrical  force  upon  it. 

A  body  charged  with  either  electrical  force  polarizes 
an  insulated  conductor  near  it,  inducing  upon  the  face 
nearest  itself  the  opposite  electrical  force.  (346.) 

Each  surface  of  a  Leyden  jar  becomes  charged  with 
the  electricity  of  the  conductor  with  which  it  is  con- 
nected, the  tinfoil  serving  to  distribute  the  electricity 
over  the  surface.  The  jar  is  discharged  by  connecting 
the  two  coatings  by  a  conductor.  (348.) 

The  action  of  points  on  charged  bodies  is  to  convey 
the  charge  off  silently  by  convective  discharge.  (350.) 

NOTE. — A  brief  account  of  Atmospheric  Electricity  will  be 
found  in  the  Appendix,  pages  256-261. 


APPENDIX. 


PHYSICS    OF    THE    ATMOSPHERE. 

TEMPERATURE   OF  THE  ATMOSPHERE. 

1.  Composition   of  the  Atmosphere. — The    atmosphere   is   a 
gaseous  ocean  surrounding  the  earth  to  the  depth  of  about  fifty 
miles.     It  is  mainly  a  mixture  of  two  gases,  nitrogen  and  oxygen^ 
in  the  proportion  of  79.1  parts  of  the  former  to  20.9  of  the  latter. 
It  contains  also  a  little  carbonic  acid  and  a  variable  amount  of 
watery  vapor. 

2.  The  Air  receives  its  Heat  directly  or  indirectly  from   the 
Sun.  — A  part  of  the  solar  rays  are  absorbed  in  passing  through 
the  atmosphere.     It   thus   becomes   warmed   directly  by  solar 
radiation.     A  part  of  the  rays  fall  upon  the  surface  of  the  earth, 
which  absorbs  them,  and  thus  becomes  heated.     This  heat  is 
then  radiated  back  again,  and  is  absorbed  by  the  air,  which  thus 
becomes  heated  by  terrestrial  radiation. 

Owing  to  the  greater  specific  heat  of  water,  the  sea  becomes 
less  heated  during  the  day  than  the  land  does.  Again,  it  is  a 
poorer  radiator  than  the  land.  Hence,  the  terrestrial  radiation 
from  the  land  is  much  greater  than  from  the  sea. 

The  watery  vapor  in  the  air  allows  the  rays  of  the  sun  to  pass 
readily  through  it  on  their  way  to  the  earth,  but  it  will  not  allow 
them  to  pass  back  again  when  they  are  radiated  from  the  earth 
as  obscure  heat.  The  sunbeams  are  thus  caught  in  a  trap  from 
which  they  cannot  escape. 

This  is  the  main  reason  why  it  is  warmer  at  the  base  of  a 
mountain  than  at  its  top,  where  the  solar  radiations  are  more 
powerful.  In  the  upper  regions  of  the  atmosphere,  there  is  less 
watery  vapor  to  absorb  the  terrestrial  radiations. 


232  APPENDIX. 

3.  The  Daily  Variation  of  Temperature.  —  The  temperature 
is  greatest,  not  at  noon,  but  two  or  three  hours  later  ;  and  least, 
not  at  midnight,  but  an  hour  or  two  before  sunrise.     During  the 
forenoon,  the  earth  receives  more  heat  than  it  radiates.     In  the 
afternoon  it  begins  to  receive  less  heat,  but  for  two   or  three 
hours  it  still  receives  more  than  it  radiates,  so  that  it  grows  hot- 
ter and  gives  out  more  heat  than  at  noon.     During  the  night,  it 
receives  no  heat  from  the  sun,  and  gives  out  less  and  less  till 
about  an  hour  before  sunrise,  when  the  heat  it  receives  from  the 
returning  sun  again  equals  what  it  radiates. 

4.  The  Distribution  of  Temperature  in  the  Atmosphere.  —  The 
highest  temperature  of  the  earth  is  found  to  be  in  an  irregular 
belt  lying  •within  the  tropics.     The  warm  belt  is  continually  shift- 
ing its  position,  passing  northward  with  the  sun  until  midsum- 
mer, and  then  southward  again  until  midwinter. 

From  the  warm  belt,  the  temperature  diminishes  towards  the 
poles.  In  the  southern  hemisphere,  which  is  nearly  all  water,  it 
shades  off  gradually  and  regularly  ;  in  the  northern,  where  there 
are  large  bodies  of  land,  the  changes  are  quite  irregular.  In 
the  summer  the  atmosphere  over  the  continents  becomes  much 
hotter  than  over  the  ocean,  owing  to  the  greater  radiation  from 
the  land ;  while  in  the  winter  the  air  over  the  continents  is  much 
colder  than  over  the  ocean,  since  the  land  has  cooled  down  faster 
than  the  sea. 

The  distribution  of  heat  is  also  modified  by  the  oceanic  cur- 
rents and  the  prevailing  winds.  The  Gulf  Stream  and  the  south- 
westerly winds  keep  the  temperature  of  western  Europe  much 
above  that  of  the  eastern  coast  of  America  in  the  same  latitude. 
For  a  similar  reason,  the  western  coast  of  America  is  warmer 
than  the  eastern  coast  of  Asia.  ^ 

ATMOSPHERIC   PRESSURE. 

5.  The  Daily  Variation  of  Atmospheric  Pressure.  —  The  ba- 
rometer shows  two  maxima  and  two  minima  of  atmospheric  pres- 
sure during  the  day :  the  former  occurring  from  nine  to  eleven, 
A.M.,  and  from  nine  to  eleven,  P.M.  ;  and  the  latter  from  three  to 


APPENDIX. 


233 


five,  A.M.,  and  from  three  to  five,  P.M.  These  variations  are 
much  more  marked  in  tropical  regions  than  elsewhere. 

6.  The  Distribution  of  Atmospheric  Pressure.  —  In  general, 
there  is  an  irregular  belt  of  low  pressure  within  the  tropics, 
bounded  on  each  side  by  a  broad  belt  of  high  pressure.  North  and 
south  of  these  are  other  belts  of  low  pressure,  while  about  each 
pole  there  is  probably  a  region  of  high  pressure.  The  belts  south 
of  the  equator  are  much  more  uniform  and  regular  than  those 
north  of  the  equator ;  as  may  be  seen  by  reference  to  Map  I.  (at 
the  end  of  the  book)  on  which  the  blue  lines  represent  pressures 
below  thirty  inches,  and  the  red  lines  thirty  inches  and  above. 

In  winter  (as  shown  in  Map  II.),  the  north  polar  region  of 
low  pressure  has  two  centres  of  least  pressure ;  one  in  the 
northern  Atlantic  near  Iceland,  and  the  other  in  the  northern 
Pacific.  At  the  same  time,  there  is  a  broad  belt  of  high  pressure 
stretching  across  Asia  and  North  America,  with  a  centre  of 
greatest  pressure  on  each  continent. 

In  summer  (as  shown  in  Map  III.),  there  are  centres  of  high 
pressure  in  the  middle  of  the  northern  Atlantic  and  Pacific ;  and 
a  broad  band  of  low  pressure  stretching  across  North  America 
and  Asia,  with  a  centre  of  least  pressure  on  each  continent. 

There  are  two  things  which  tend  to  diminish  the  atmospheric 
pressure ;  high  temperature  and  great  humidity.  High  tempera- 
ture causes  the  air  to  expand,  rise,  and  flow  away  to  colder 
regions.  Great  humidity  diminishes  the  density  of  the  air. 
Humid  air  therefore  rises,  but  in  rising,  it  becomes  cooled,  and 
a  part  of  its  moisture  falls  as  rain.  In  this  condensation,  a  large 
amount  of  heat  is  given  out,  which  again  raises  the  temperature 
of  the  air,  and  causes  it  to  expand  still  more. 

Now  in  the  tropics  there  is  an  excess  of  both  heat  and  moisture. 
The  air  therefore  rises  and  flows  over  towards  the  north  and 
south,  giving  rise  to  the  belt  of  low  pressure  bounded  by  belts  of 
high  pressure. 

Again,  in  the  regions  north  and  south  of  these  belts  of  low 
pressure,  the  air  is  highly  charged  with  moisture  brought  thither 
by  the  prevailing  winds,  and  continually  condensed  in  rain. 
Here  also,  then,  the  air  rises  and  flows  over  towards  the  north 


234  APPENDIX. 

and  south,  producing  a  region  of  high  pressure  towards  the 
poles  and  increasing  the  pressure  in  the  belts  towards  the 
equator. 

The  irregularity  in  the  belts  of  pressure  in  the  northern  hem- 
isphere is  caused  by  the  continents.  In  the  summer,  both  North 
America  and  Asia  become  excessively  heated,  while  the  adjacent 
seas  are  comparatively  cool.  Hence  the  air  pours  over  from  the 
land  to  the  sea,  giving  rise  to  low  pressure  on  the  continents  and 
high  pressure  on  the  oceans.  The  more  completely  the  sea  is 
shut  in  by  the  heated  land,  as  in  the  northern  Atlantic,  the 
greater  the  atmospheric  pressure  upon  it.  It  is  also  this  exces- 
sive heat  of  the  northern  continents  in  summer  which  causes 
the  great  pressure  upon  the  southern  hemisphere  at  the  same 
season.  (See  Map  III.) 

In  winter,  the  conditions  are  reversed.  The  land  becomes 
excessively  cold,  and  the  air  over  it  dense  and  contracted.  The 
warmer  air  from  the  sea  now  pours  over  upon  the  land,  causing 
the  high  pressure  in  North  America  and  Asia,  and  the  low 
pressure  in  the  northern  Atlantic  and  Pacific.  (See  Map  II.) 

WINDS. 

7.  Cause  of  Winds.  —  Winds  are  currents  of  air,  and  are 
directly  caused  by  atmospheric  pressure.  If  two  neighboring 
regions  come  to  be  of  very  unequal  temperature,  the  lighter  air 
of  the  warmer  region  will  rise  and  flow  over  to  the  colder  region, 
while  the  heavier  air  of  the  colder  region  will  flow  in  below  to 
supply  its  place.  Thus  we  always  have  a  surface  wind  blowing 
from  a  region  of  lower  temperature  and  high  pressure  towards 
one  of  higher  temperature  and  low  pressure,  and  an  upper  wind 
blowing  in  the  opposite  direction.  We  have  an  illustration  of 
this  in  the  wind  which  always  sets  in  from  every  dii*ection  tow- 
ards a  large  fire.  We  have  another  in  the  land  and  sea  breezes. 
On  the  sea-coast  a  breeze  sets  in  from  the  sea  in  the  morning. 
At  first  a  mere  breathing,  it  gradually  rises  to  a  stiff  breeze  in 
the  heat  of  the  day,  and  again  sinks  to  a  calm  towards  evening. 
Soon  after,  a  breeze  springs  up  from  the  land,  and  blows  strongly 


APPENDIX.  235 

seaward  during  the  night,  dying  away  towards  morning,  when 
the  sea-breeze  begins  once  more.  These  breezes  are  especially 
marked  in  tropical  regions,  where  the  difference  of  temperature 
on  land  and  sea  is  greatest. 

8.  Trade-  Winds.  — While  the  air  above  is  flowing  north  and 
south  from  the  tropical  belt  of  low  pressure,  a  surface  wind  will 
set   in   from   the  region  of  high  pressure  to  supply  its  place. 
Were  the  earth  at  rest,  these  surface  winds  would  blow  directly 
from  the  north  and  south  towards  the  equator.     But  the  earth 
is  rotating  from  west  to  east,  and  objects  on  the  surface  at  the 
equator  are  carried  round  towards  the  east  at  the  rate  of  about 
17  miles  a  minute.     But  as  we  go  away  from  the  equator,  this 
velocity  diminishes,  so  that  in  latitude  60°  it  is  only  8i  miles  a 
minute,  and  at  the  poles  it  is  nothing.    The  wind,  then,  blowing 
towards  the  equator,  is  contimially  coming  to  places  'which  have 
greater  velocity  east-ward  than  itself,  and  therefore  lags  behind 
and  appears  to  move  •west-ward.     This,  combined  •with  its  motion 
towards  the  equator,  makes  the  surface  wind  north  of  the  equa- 
tor a  north-east  wind,  and  the  one  south  of  the  equator  a  south- 
east wind.     These  winds  blow  with  great  steadiness  and  con- 
stancy, and  from  the  service  they  render  to  commerce  are  called 
trade--winds. 

In  mid-ocean  in  the  Atlantic,  the  north  trades  prevail  between 
latitudes  9°  and  30°,  and  in  the  Pacific,  between  latitudes  9°  and 
26° ;  and  the  south  trades  in  the  Atlantic,  between  latitudes  4° 
north  and  22°  south,  and  in  the  Pacific  between  latitudes  4° 
north  and  23^°  south.  These  limits  are,  however,  not  station- 
ary, but  follow  the  sun,  advancing  northward  from  January  to 
June,  and  retreating  southward  from  July  to  December. 

9.  Region  of  Calms.  — The  region  of  calms  is  a  belt  of  about 
4°  or  5°  in  breadth,  stretching  across  the  Atlantic  and  the  Pacific, 
generally  parallel  to  the  equator.    It  is  marked  by  a  lower  atmos- 
pheric pressure  than  is  found  to  the  north  and  to  the  south  of  it 
in  the  regions  of  the  trade-winds.     It  is  also  characterized  by 
the  daily  occurrence  of  heavy  rains  and  severe  thunder-storms. 
The  position  of  the  belt  varies  with  the  sun. 

There  are  two  other  regions  or  belts  of  calms  at  the  limits  of 


236  APPENDIX. 

the  north  and  south  trades.  Except  in  the  Pacific  Ocean,  these 
belts  are  either  broken  up,  so  as  to  appear  only  in  patches,  or 
are  completely  obliterated  by  the  disturbing  influences  arising 
from  the  unequal  distribution  of  land  and  water.  Of  these 
smaller  regions  of  calms,  the  most  interesting  is  that  marked 
out  by  the  high  pressures  in  the  North  Atlantic.  This  is  the 
region  of  the  Sargasso  Sea,  which  is  thus  characterized  not 
only  by  its  still  waters,  but  also  by  its  still  atmosphere.  A  sim- 
ilar region  of  calms  exists  in  the  South  Atlantic.  These  calms 
are  well  known  to  sailors. 

10.  Winds  in   Middle  Latitudes.  —  Surface   winds   will  flow 
from  the  belts  of  high  pressure,  not  only  towards  the  equatorial 
belt  of  low  pressure,  but  also  towards  the  belts  of  low  pressure 
on  the  other  side.     These  currents  are  continually  coming  to 
places  which  have  less  velocity  eastward  than  their  own,  and 
therefore  appear  to  move  eastward.     This,  combined  with  their 
motion  from  the  equator,  tends  to  make  them  south-west  winds 
in  northern  latitudes  and  north-west  winds  in  southern  latitudes. 
These  are  the  prevailing  winds  in  these  regions,  but,  owing  to 
various  disturbing  causes,  they  are  much  less  uniform  and  con- 
stant  than  the  trade-winds.     This  is  especially  true  of  the  north- 
ern region. 

There  will  also  be  surface  winds  blowing  from  the  poles 
towards  these  same  belts  of  low  pressure.  Of  course,  there 
will  be  upper  currents  in  opposite  directions  to  all  these  surface 
winds. 

11.  Winds  of  the  Northern  Atlantic.  —  It  is  found  that  wher- 
ever there  is  a  centre  of  low  pressure,  the  winds  blow  towards  it, 
not  directly,  but  spirally,  and  somewhat  to  the  right  of  it.     We 
have  an  illustration  of  this  in  the  winter  winds  of  the  northern 
Atlantic,  when  there  is  a  centre  of  low  pressure  near  Iceland. 
Along  the  North  American  coast  the  prevailing  winds  are  from 
the  N.W.     At  the  more  northern  places  the  general  direction  is 
more  northerly,  while  farther  south  it  is  more  westerly.     In  the 
Atlantic,  between  Great  Britain  and  America,  the  direction  is 
nearly  S.W. ;  this  is  also  nearly  the  direction  in  France,  Bel- 
gium, and  the  south  of  England.     At  Dublin,  and  in  the  south 


APPENDIX.  237 

of  Scotland,  it  is  about  W.S.W. ;  at  Copenhagen  it  is  S.S.W. ; 
at  St.  Petersburg,  it  is  nearly  S. ;  and  at  Hammerfest,  near  the 
North  Cape  in  Norway,  it  is  S.S.E.  We  thus  see  that  the  whole 
atmosphere  flows  in  towards  and  upon  the  region  of  low  pressure 
round  Iceland,  —  not  directly  towards  the  region,  of  lowest  pres- 
sure, but  in  a  direction  a  little  to  the  right  of  it.  We  can  now 
understand  why  it  is  that  the  prevailing  winds  in  North  Amer- 
ica at  this  season  are  N.W.,  while  in  Greenland  and  in  Great 
Britain  a  N.W.  wind  is  scarcely  known. 

It  is  mainly  to  this  low  pressure  which  draws  over  Great 
Britain  the  S.W.  winds  from  the  warm  waters  of  the  Atlantic 
that  this  island  owes  its  mild,  open,  and  rainy  winters.  It  is 
the  same  pressure  which  gives  Russia  and  Central  Europe  their 
severe  winters,  since  on  account  of  it  a  slow,  steady  air-current 
from  the  cold  regions  of  Northern  Asia  is  drawn  westward  over 
those  parts  of  Europe.  Finally,  the  same  low  pressure  draws 
over  British  America  and  the  United  States,  by  the  N.W.  wind, 
the  cold,  dry  currents  of  the  polar  regions.  In  the  State  of 
Maine,  the  mean  January  temperature  is  about  23°,  whilst  on 
the  coast  of  England,  10°  farther  north,  it  is  as  high  as  40°. 

12.  Monsoons. — The  term  monsoon,  derived  from  the  Arabic 
word  mausim  (a  set  time  or  season  of  the  year)  has  been  long 
applied  to  the  prevailing  winds  in  the  Indian  Ocean,  which 
blow  from  the  S.W.  from  April  to  October,  and  from  the  N.E., 
or  opposite  direction,  from  October  to  April.  During  the  sum- 
mer, when  the  sun  is  north  of  the  equator,  the  continent  of 
Asia  becomes  heated  to  a  much  greater  degree  than  the  Indian 
Ocean,  which  in  its  turn  is  warmer  than  Australia  and  South 
Africa.  Hence,  as  the  heated  air  of  Southern  Asia  expands  and 
rises,  and  the  pressure  is  thereby  reduced  nearly  half  an  inch 
below  the  average,  colder  air  from  the  S.  flows  in  to  take  its 
place,  and  thus  a  general  movement  of  the  atmosphere  of  the 
Indian  Ocean  sets  in  towards  the  N.,  giving  a  southerly  direc- 
tion to  the  wind.  But  as  the  wind  comes  from  parts  of  the 
globe  which  revolve  quicker  to  those  which  revolve  more  slow- 
ly, it  gets  a  westerly  direction.  The  combination  of  these  two 
directions  results  in  the  S.  W.  monsoon,  which  accordingly  pre- 


238  APPENDIX. 

vails  there  in  summer.  Since,  during  winter,  when  the  sun  is 
south  of  the  equator,  Asia  is  colder  than  the  Indian  Ocean,  and 
the  pressure  is  thereby  increased  nearly  half  an  inch  above  the 
average,  a  general  movement  of  the  atmosphere  sets  in  towards 
the  S.  and  W.  As  this  is  the  same  direction  as  the  ordinary 
trade-wind,  the  result  during  winter  is  not  to  change  the  direc- 
tion of  that  wind,  but  only  to  increase  its  velocity. 

Similar,  though  less  strongly  marked  monsoons  prevail  off 
the  coasts  of  Upper  Guinea  in  Africa,  and  Mexico  in  America. 


STORMS. 

13.  Storms.  —  Besides  the  general  atmospheric  disturbances 
already  described,  there  are  local  disturbances  of  the  same  kind, 
called  storms.  When  the  air  over  any  considerable  tract  be- 
comes excessively  heated  and  humid,  it  rises  and  overflows, 
producing  a  local  centre  of  least  pressure.  Surface  winds  set 
in  towards  this  centre  from  all  sides  in  a  spiral  direction  /  as 
the  humid  air  rises  it  becomes  cooled,  and  its  moisture  is  con- 
densed as  rain  or  snow.  A  large  amount  of  heat  is  set  free, 
which  causes  the  air  to  expand  still  more.  Sometimes  these 
storms  remain  stationary,  but  they  generally  move  forward  in 
an  easterly  direction. 

The  storms  of  North  America  usually  have  their  rise  in  the 
region  east  of  the  Rocky  Mountains,  travel  eastward  towards 
the  coast,  and  cross  the  Atlantic.  They  are  preceded  by  a  high 
temperature  and  a  moist  air,  and  followed  by  a  low  temperature 
and  a  dry  air.  When  the  storm  is  approaching,  the  wind  sets 
in  from  the  east,  and*  there  is  usually  the  heaviest  fall  of  rain 
before  the  centre  of  the  storm  arrives.  In  this  centre  there  is 
usually  a  calm,  and  often  considerable  clear  sky.  As  the  centre 
passes,  the  wind  suddenly  veers  round  to  the  west,  and  a  short, 
heavy  fall  of  rain  follows ;  the  temperature  rapidly  falls,  and 
the  barometer  rapidly  rises.  When  the  centre  of  the  storm 
passes  to  the  north,  the  wind  sets  in  from  the  south-east,  and 
veers  round  by  the  south  to  the  south-west.  When  the  centre 


APPENDIX. 


239 


of  the  storm  passes  to  the  south,  the  wind  sets  in  from  the 
north-east,  and  veers  round  by  the  north  to  the  north-west. 

When  a  great  storm  begins  near  the  Mississippi,  the  wind  at 
St.  Louis  will  be  easterly,  while  farther  east  it  will  be  westerly. 
This  easterly  wind  travels  eastward  with  the  storm ;  that  is,  in 
a  direction  opposite  to  that  in  which  it  blows.  The  westerly 
wind  which  follows  the  storm  travels  along  with  it;  that  is,  in 
the  same  direction  as  that  in  which  it  blows. 

The  storms  of  America  are  usually  very  long  in  a  north  and 
south  direction,  and  travel  side  foremost ;  while  the  storms  of 
Europe  are  usually  circular  or  slightly  oblong  in  the  direction 
of  their  motion.  The  latter  are  followed  by  less  depression  of 
temperature  than  those  of  America. 

Tornadoes  are  very  violent  storms,  usually  of  small  dimen- 
sions. Here,  as  in  other  storms,  the  wind  sets  in  spirally 
towards  the  region  of  least  pressure,  which  is  also  the  centre 
of  the  storm. 

14.  Whirlwinds.  —  Whirlwinds  are  very  different  from  the 
storms  already  described.  They  seldom  last  longer  than  a 
Fig.  197- 


minute,   sometimes   only  a  few  seconds ;    their  breadth   varies 
from  twenty  to  a-  few  hundred  yards ;  their  course  seldom  en- 


240 


APPENDIX. 


ceeds  25  miles  in  length ;  and,  while  they  last,  the  changes  of 
the  'wind  are  sudden  and  violent.  The  direction  of  the  eddy 
of  the  whirlwind  is  not  uniform,  as  in  a  storm,  but  depends  on 
the  direction  of  the  stronger  of  the  two  winds  which  give  rise 
to  it.  Thus,  suppose  a  whirlwind  be  produced  by  the  rushing 
of  a  north  wind  against  a  south  wind :  then,  if  the  north  wind 
be  the  stronger  and  on  the  west,  the  whirl  will  be  in  the  direc- 
tion of  the  hands  of  a  watch ;  but  if  the  south  wind  be  the 
stronger,  the  eddy  will  turn  in  the  opposite  direction. 

Whirlwinds  are  often  originated  in  the  tropics  during  the 
hot  season,  especially  in  flat,  sandy  deserts,  which,  becoming 
unequally  heated  by  the  sun,  give  rise  to  numerous  ascending 
columns  of  air.  In  their  contact  with  each  other,  these  ascend- 
ing currents  give  rise  to  eddies,  thus  producing  whirlwinds 
which  carry  up  with  them  clouds  of  dust.  Of  this  description 

Fig.  198. 


are  the  dust-'whirl'winds  of  India,  illustrated  in  Figures  197  and 
198.     The  large  arrows,  in  Figure  198,  show  the  rotation  of  the 


APPENDIX.  241 

whole  whirlwind  round  its  axis,  while  the  small  arrows  show 
the  rotation  of  each  column  round  its  own  axis.  Figure  197 
shows  the  general  appearance  of  a  dust-whirlwind  as  seen  from 
a  distance.  A  dust-storm  is  caused  by  a  number  of  whirlwind 
columns  moving-  together  over  the  earth.  The  storm  generally 
comes  on  without  warning  from  any  direction,  and  the  barom- 
eter is  said  not  to  be  perceptibly  affected  by  it.  A  low  bank  of 
dark  cloud  is  seen  in  the  horizon,  which  rapidly  increases,  and, 
before  the  spectator  is  aware,  the  storm  bursts  upon  him,  wrap- 
ping every  thing  in  midnight  darkness.  An  enormous  quantity 
of  dust  is  whirled  aloft,  which  is  sometimes  broken  into  distinct 
columns,  each  whirling  on  its  axis.  Violent  gusts  or  squalls 
succeed  each  other  at  intervals,  which  gradually  become  weaker ; 
and,  at  the  close  of  the  storm,  a  fall  of  rain  generally  takes 
place.  The  air  is  often  highly  electrical,  arising  probably  from 
the  friction  of  the  dust-laden  currents  against  each  other.  The 
Simoom  may  be  regarded  as  in  part  a  whirlwind,  or  a  succes- 
sion of  whirlwinds  of  this  description.  Sir  S.  W.  Baker  thus 
graphically  describes  the  behavior  of  the  dust-whirlwinds  which 
occur  in  Nubia  in  April,  May,  and  June :  "I  have  frequently 
seen  many  such  columns  at  the  same  time  in  the  boundless 
desert,  all  travelling  or  waltzing  in  various  directions,  at  the 
fitful  choice  of  each  whirlwind ;  this  vagrancy  of  character  is  an 
undoubted  proof  to  the  Arab  mind  of  their  independent  and 
diabolical  character." 

Extensive  fires,  such  as  the  burning  of  the  prairies  in  Ameri- 
ca, and  volcanic  eruptions,  also  cause  whirlwinds  by  the  upward 
current  produced  by  the  heated  air;  and  these,  as  well  as  the 
other  whirlwinds  already  mentioned,  are  occasionally  accom- 
panied with  rain  and  electrical  displays. 

15.  Waterspouts.  —  Waterspouts  are  whirl-winds  occurring 
over  the  sea  or  over  sheets  of  fresh  water.  When  fully  formed, 
they  appear  as  tall  pillars  stretching  from  the  sea  upward  to  the 
clouds,  and  whirling  round  their  axes,  like  the  dust-whirlwinds. 
The  sea  is  tossed  into  violent  agitation  round  their  bases  as 
they  career  onward.  The  danger  arising  from  them  consists  in 
the  enormous  velocity  of  the  wind,  and  the  sudden  changes 

16 


242  APPENDIX. 

in  its  direction.  It  is  a  popular  fallacy  that  the  water  of  the  sea 
is  sucked  up  by  them,  it  being  only  the  spray  from  the  broken 
waves  that  is  carried  up  by  the  whirling  vortex.  This  is  proved 
by  the  fact  that  the  water  poured  down  on  the  decks  of  vessels 
from  waterspouts  is  either  fresh  or  only  slightly  brackish. 


THE  MOISTURE   OF  THE   ATMOSPHERE. 

16.  The  Two  Atmospheres  of  Air  and  Vapor. — The  gaseous 
envelope  of  the  earth  may  be  considered  as  made  up  of  two  dis- 
tinct atmospheres,  —  one  of  dry  air,  and  one  of  vapor.     The  dry 
air  is  always  a  gas,  and  its  quantity  is  constant  from  year  to 
year;  but  the  vapor  of  water  does  not  always  remain  in  the 
gaseous  state,  and  the  quantity  in  the  atmosphere  varies  every 
instant. 

17.  Evaporation. — Vapor  is  continually  passing  into  the  air 
from  the  surface  of  water  and  moist  bodies  at  all  temperatures 
by  the  silent  process  of  evaporation.     Evaporation  also  takes 
place  from  the  surface  of  snow  and  ice.     The  atmosphere  can 
contain  only  a  certain  amount  of  vapor,  according  to  the  tem- 
perature ;  hence,  when  it  is  saturated  with  moisture,  evaporation 
ceases.     Conversely,  evaporation  will  be  greatest  when  the  air  is 
perfectly  free  from  vapor.     Since  atmospheric  currents  remove 
the  saturated  air  and  substitute  dry  air,  evaporation  is  muck 
more  rapid  in  windy  than  in  calm  weather. 

18.  Loss  of  Heat  by  Evaporation.  —  We   have   learned  that 
when  a  liquid  passes  into  the  gaseous  form,  a  large  quantity 
of  heat  becomes  latent;   and  that  this   heat  becomes  sensible 
again  when  the  vapor  returns  to  the  liquid  state.     The  ocean 
loses  more  heat  from   evaporation  than   the  land,   because    the 
quantity  evaporated  from  its  surface  is  much  greater.     Again, 
since  more  rain  falls  on  land  than  on  sea,  especially  in  hilly  and 
mountainous  countries,  the  temperature  of  the  air  over  the  land 
will  be  still  further  raised  by  the  heat  thus  given  out.     This  is 
one  of  the  reasons  why  the  mean  temperature  of  the  northern 
hemisphere  is  higher  than  that  of  the  southern. 


APPENDIX.  243 

It  is  for  this  reason  that  the  sensible  temperature  depends  on 
the  humidity  of  the  air.  Dry  air  promotes  evaporation  from  the 
surface  of  the  body,  and  seems  cold ;  while  moist  air  impedes 
this  evaporation,  and  seems  warm.  When  the  air  is  both  hot 
and  moist,  as  in  the  dog-days,  it  is  peculiarly  oppressive.  It  is 
because  the  winds  promote  evaporation  that  the 'air  seems  cooler 
on  a  windy  day  than  on  a  still  one,  though  the  temperature  may 
be  the  same. 

19.  Deiv. — After  the  sun  has   set,  the  earth  is  continually 
radiating   heat   into   space,   and  is  receiving  little  or  none  in 
return.     As  it  cools  down,  it  cools  the  layer  of  air  nearest  to  it, 
and  causes  it  to  deposit  its  moisture  in  the  form  of  deiu.     In  the 
same  way,  in  hot  weather,  moisture  collects  on  the  outside  of  a 
pitcher  of  ice-water.     The  cold  pitcher  cools  the  air  nearest  it, 
and  compels  it  to  give  up  a  part  qf  its  moisture. 

Every  one  has  noticed  that  dew  collects  on  some  substances 
more  readily  than  on  others.  This  is  because  they  are  better 
radiators,  and  therefore  cool  sooner. 

Dew  does  not  collect  on  a  cloudy  night,  or  under  a  roof  or 
shed,  because  the  heat  is  sent  back  by  the  clouds  and  the  roof 
as  fast  as  it  is  radiated  from  the  earth. 

There  is  no  dew  on  a  very  windy  night,  because  the  layer  of 
air  near  the  earth  is  continually  changing,  and  does  not  become 
cool  enough  to  give  up  its  moisture. 

20.  Detv-point.  —  The    ascertaining  of  the   dew-point   is    of 
great  practical  importance,  particularly  to  horticulturists,  since 
//  shows  the  point  near  which  the  temperature  during  the  night 
'will  cease  to  fall.     For  when  the  air  has  been  cooled  down  by 
radiation  to  this  point,  dew  is  deposited,  heat  is  given  out,  and 
the  temperature  of  the  air  rises.    But  as  the  cooling  by  radiation 
proceeds,  the  air  again  falls  to,  or  slightly  under,  the  dew-point; 
dew  is  now  again  deposited,  heat  liberated,  and  the  temperature 
raised.     Thus  the  temperature  of  the  air  in  contact  with  plants 
and  other  radiating  surfaces  may  be  considered  as  gently  oscil- 
lating about  the  dew-point.     For  if  it  rises  higher,  the  loss  of 
heat  by  radiation  soon  lowers  it ;  and  if  it  falls  lower  by  ever  so 
little,  the  heat  liberated  by  the  formation  of  dew  soon  raises  it. 


244  APPENDIX. 

The  dew-point,  then,  determines  the  lowest  temperature  of  the 
night;  and  if  this  point  be  found  by  means  of  the  hygrodeik, 
the  approach  of  low  temperature,  or  of  frost,  may  be  foreseen 
and  provided  against. 

MISTS,   FOGS,  AND   CLOUDS. 

21.  Mists  and  Fogs.  —  Mists  and  fogs  are  visible  valors  float- 
ing in  the  air  near  the  surface  of  the  earth.  They  are  produced 
in  various  ways,  —  by  the  mixing  of  cold  air  with  air  that  is 
warm  and  moist,  or  by  whatever  tends  to  lower  the  temperature 
of  the  air  below  the  dew-point. 

During  a  calm,  clear  night,  when  the  air  over  a  level  country 
has  been  cooled  by  radiation,  and  dew  begins  to  be  deposited, 
the  portion  of  the  air  in  contact  with  the  ground  is  lowered  to 
the  dew-point,  and  thus  becomes  colder  than  the  air  above  it. 
Since  there  is  nothing  to  disturb  the  equilibrium  and  give  rise 
to  currents  of  air,  and  no  cause  in  operation  which  can  reduce 
the  temperature  much  below  the  point  of  saturation,  the  air 
within  a  few  feet  of  the  surface  remains  free  from  mist  or  fog. 
But  if  the  ground  slopes,  the  cold  air,  being  heavier,  will  flow 
down  and  fill  the  low»r  grounds  ;  and  since  it  is  colder  than  the 
saturated  air  which  it  meets  with  in  its  course,  it  will  reduce  its 
temperature  below  the  point  of  saturation,  and  thus  produce 


When  an  oceanic  current  meets  a  shoal  in  its  course,  the  cold 
water  of  the  lower  depths  is  brought  to  the  surface,  and  in  all 
cases  where  its  temperature  is  lower  than  the  dew-point  of  the 
air,  fogs  are  formed  over  the  shoal.  For  a  similar  reason  ice- 
bergs are  frequently  enveloped  in  fogs.  In  like  manner,  mist  is 
sometimes  seen  to  rise  from  rivers  whose  temperature  is  lower 
than  that  of  the  air.  Thus  the  waters  of  the  Swiss  rivers  which 
issue  from  the  cold  glaciers  cool  the  air  in  contact  with  them 
below  the  point  of  saturation,  and  produce  mist.  So,  also,  such 
rivers  as  the  Mississippi,  which  flow  directly  into  warmer  lati- 
tudes, and  are  therefore  colder  than  the  air  above  them,  are 
often  covered  with  mist  or  fogs. 


APPENDIX.  245 

When  rivers  are  considerably  warmer  than  the  air,  the  more 
rapid  evaporation  from  the  warm  water  pours  more  vapor  into 
the  atmosphere  than  it  can  hold,  and  the  surplus  is  condensed 
into  mist.  Thus  deep  lakes,  and  rivers  flowing  out  of  them,  are 
in  winter  generally  much  warmer  than  the  air,  and  hence,  when 
the  air  is  cold  and  moist,  they  are  covered  with  fogs. 

The  densest  fogs  occur  during  the  cold  months  in  large  towns 
built  on  rivers,  since  the  causes  which  produce  fogs  are  then 
most  active.  The  peculiar  denseness  of  the  London  November 
fogs  is  caused  by  the  warmth  of  the  river-bed,  and  it  is  in- 
creased by  the  sources  of  artificial  heat  which  London  affords ; 
and  since  the  temperature  is  falling  everywhere,  and  the  air  is 
very  moist,  its  vapor  is  quickly  and  copiously  condensed  by  the 
cold  easterly  winds  of  the  season. 

In  all  these  cases,  the  fogs  do  not  extend  very  widely  nor  rise 
very  high.  There  are,  however,  other  fogs  that  spread  over 
large  districts,  like  the  fogs  which  often  accompany  the  break- 
ing up  of  frosts  in  winter.  When  the  moist  south-west  wind 
has  gained  the  ascendency,  it  is  chilled  by  contact  with  the  cold 
ground,  and  its  abundant  vapor  condensed  into  mist. 

Mountains  are  frequently  covered  with  mist.  As  the  warm 
air  is  driven  up  the  slopes  of  the  mountain  by  the  wind,  it  be- 
comes gradually  colder,  until  at  last  its  moisture  is  condensed. 
Mists  often  appear  soonest  on  the  parts  of  hills  covered  with 
trees,  especially  when  the  mist  begins  to  form  after  mid-day, 
because  then  the  temperature  of  the  trees  is  lower  than  that  of 
the  grassy  slopes.  Occasionally  the  summit  of  a  hill  or  an 
isolated  peak  is  wrapped  in  mist,  while  elsewhere  the  air  is 
clear;  and  though  a  breeze  be  blowing  over  the  hill,  still 

"  Overhead 
The  light  cloud  smoulders  on  the  summer  crag," 

apparently  motionless  and  unchanged.  This  is  easily  explained. 
The  temperature  at  the  top  is  below  the  dew-point  of  the  atmos- 
pheric current.  Hence  when  the  air  rises  to  this  region,  its 
moisture  is  condensed  into  mist.  This  is  borne  forward  over 
the  hill  and  down  the  other  side,  acquiring  heat  as  it  descends 


246  APPENDIX. 

till  it  is  again  dissolved  and  disappears.  Meanwhile  its  place  is 
constantly  supplied  by  fresh  condensation,  as  the  current  rises 
to  the  summit.  Thus,  though  the  mist  on  the  top  of  the  hill 
appears  motionless  and  unchanged,  it  is  continually  undergoing 
renewal. 

/"22.  Clouds.  —  Clouds  are  visible  vapors  floating  in  the  air  at  a 
''considerable  height',  thus  differing  from  mists  and  fogs,  which 
float  near  the  surface.  Both  arise  from  the  same  causes. 

During  the  warmest  part  of  the  day,  when  evaporation  is 
greatest,  warm,  moist  air-currents  are  constantly  ascending  from 
the  earth.  As  they  rise  in  succession,  the  moist  air  is  pushed 
high  up  into  the  atmosphere,  and  loses  heat  by  expansion  until 
it  can  no  longer  retain  its  moisture.  Hence  condensation  takes 
place,  and  a  cloud  is  formed,  which  increases  in  bulk  as  long 
as  the  air  continues  to  ascend.  But  as  the  day  declines,  and 
evaporation  is  checked,  the  ascending  current  ceases ;  and,  the 
temperature  falling  from  the  earth's  surface  upwards,  the  lower 
stratum  of  air  contracts.  Consequently  the  whole  mass  of  air 
begins  to  descend,  and  the  clouds  are  then  dissolved  by  the 
warmth  they  acquire  in  falling  to  lower  levels.  The  whole  of 
this  process  is  frequently  seen  on  a  warm  summer  day.  In  the 
morning  the  sky  is  cloudless,  or  nearly  so ;  as  the  heat  becomes 
greater,  clouds  begin  to  form  before  noon,  and  gradually  in- 
crease in  numbers  and  size;  but,  as  the  heat  diminishes,  they 
contract  their  dimensions,  and  gather  round  the  setting  sun,  lit 
up  with  the  fiery  splendors  of  his  beams.  In  a  short  time  they 
disappear,  and  the  stars  come  out,  shining  in  a  cloudless  sky. 

The  whole  atmosphere,  to  a  great  height,  is  constantly  trav- 
ersed by  many  aerial  currents,  one  above  another,  and  flowing  in 
different  and  frequently  in  opposite  directions.  Masses  of  air  of 
different  temperatures  are  thus  frequently  brought  together; 
and  since,  when  mingled,  they  cannot  hold  the  same  quantity 
of  vapor  that  each  could  retain  before  they  were  united,  the 
excess  is  condensed  into  cloud. 

But  again,  when  a  dry  and  heavy  wind  takes  the  place  of  a 
moist  and  light  wind,  it  generally  edges  itself  beneath  the  moist 
wind  and  forces  it,  as  with  a  wedge,  into  the  upper  regions  of 


APPENDIX.  247 

the  atmosphere.  There  its  vapor  is  soon  condensed,  and  dense 
black  clouds,  often  heavily  charged  with  rain,  are  formed. 

Currents  of  air  driven  up  the  sloping  sides  of  hills  and  moun- 
tains by  the  winds  often  cause  the  formation  of  clouds  (21). 

How  are  the  clouds  suspended  in  the  air?  The  cloud  itself 
may  appear  stationary  or  suspended  (21),  but  the  particles  of 
which  it  is  composed  are  undergoing  constant  renewal.  The  pa.r- 
ticles  are  upheld  by  the  force  of  the  ascending  current  in  which 
they  are  formed ;  but  when  that  current  ceases  to  rise,  or  when 
they  become  separated  from  it,  they  begin  to  fall  through  the 
air  by  their  own  weight,  till  they  melt  away  and  are  dissolved  in 
the  higher  temperature  into  which  they  fall.  Hence,  every  cloud 
is  either  a  forming  cloud,  or  a  dissolving  cloud.  While  it  is  con- 
nected with  an  ascending  current,  it  increases  in  size,  is  dense  at 
the  top,  and  -well  defined  in  outline;  but  when  the  ascending 
current  ceases,  the  cloud  diminishes  in  size  and  density. 

23.  Classification  of  Clouds.  —  Clouds  are  divided   into  seven 
kinds;    three   being   simple,    the   cirrus,    the   cumulus,    and    the 
stratus;  and  four  intermediate  or  compound,  the  cirro-cumulus, 
the  cirro-stratus,  the  cumulo-stratus,  and  the  cumulo-cirro-stratus 
or  nimbus. 

These  forms  of  clouds,  with  the  exception  of  the  nimbus,  are 
represented  in  the  plate  on  page  249.  The  one  marked  by  one 
bird  is  the  cirrus;  by  two  birds,  the  cirro-cumulus;  by  three,  the 
cirro-stratus;  by  four,  the  cumulus;  by  five>  the  cumulo-stratus; 
by  six,  the  stratus. 

24.  Cirrus  Cloud.  — The   cirrus  (or  curl)  cloud   consists  of 
parallel,  -wavy,  or  diverging  fibres  which  may  increase  in  any 
or  in  all  directions.     Of  all  clouds  it  has  the  least  density,  the 
greatest  elevation,  and  the  greatest  variety  of  figure.     It  is  the 
cloud  first  seen  after  serene  weather,  appearing  as  slender  fila- 
ments stretching  like  white  lines  pencilled  across  the  blue  sky, 
and  thence  spreading  in  one  or  more  directions,  laterally,  or 
upward,  or  downward.     Sometimes  the  thin  lines  of  cloud  are 
arranged  parallel  to  each  other,  the  lines  lying  in  the  northern 
hemisphere  from  north  to  south,  or  from  south-west  to  north- 
east ;  sometimes  they  diverge  from  each  other  in  the  form  of  the 

\ 


248  APPENDIX. 

tail  of  a  horse;  while  at  other  times  they  cross  each  other  in 
different  ways  like  rich,  delicate  lace-work.  It  is  probable  that 
the  fine  particles  of  which  this  cloud  is  composed  are  minute 
crystals  of  ice  or  snow-flakes.  The  duration  of  the  cirrus  varies 
from  a  few  minutes  to  many  hours.  It  remains  for  a  short  time 
when  formed  in  the  lower  parts  of  the  atmosphere  and  near 
other  clouds,  and  longest  when  it  appears  alone  in  the  sky  and 
at  a  great  height. 

25.  Cumulus.  —  This  name  is   applied  to  convex  or  conical 
heaps  of  clouds   increasing  upwards   from   a  horizontal   base. 
They  are  usually  of  a  very  dense  structure ;   are  formed  in  the 
lower  regions   of  the   atmosphere;    and    are   carried   along  in 
the  current  next  the  earth.     The  cumulus  has  been  well  called 
the  cloud  of  the  day,  being  caused  by  the  ascending  currents  of 
warm  air  which  rise  from  the  heated  ground.     Its  beginning  is 
the  little  cloud  not  bigger  than  a  man's  hand,  which  is  the 
nucleus  round  which  it  increases.     The  lower  surface  remains 
roughly  horizontal,  while  the  upper  rises  into  towering  heaps, 
which  may  continue  comparatively  small,  or  swell  into  a  size 
far  exceeding  that  of 'mountains. 

When  these  clouds  are  of  moderate  height  and  size,  of  a  well- 
defined  curved  outline,  and  appear  only  during  the  heat  of  the 
day,  they  indicate  a  continuance  of  fair  weather.  But  when 
they  increase  with  great  rapidity,  sink  down  into  the  lower 
parts  of  the  atmosphere,  and  do  not  disappear  towards  even- 
ing, rain  may  be  expected.  If  loose  fleecy  patches  of  cloud 
begin  to  be  thrown  out  from  their  surfaces,  the  rain  is  near 
at  hand. 

26.  Stratus. — The  stratus,  as  its  name  implies,  is  a  widely- 
extended,  continuous  layer  or  sheet  of  cloud,  increasing  from 
below  upwards.     It  is,  besides,  the  lowest  sort  of  cloud,  its  lower 
surface  commonly  resting  on  the  earth.     The  stratus  may  be 
called  the  cloud  of  night,  since  it  generally  forms  about  sunset, 
grows  denser  during  the  night,  and  disappears  about  sunrise. 
It  is  caused  by  the  vapors  which  rise  during  the  day,  but  towards 
evening  fall  to  the  earth  with  the  falling  temperature.     Since 
during  night  the  cooling  of  the  air  begins  on  the  ground,  the 


APPENDIX. 


249 


250  APPENDIX. 

stratus  first  appears  like  a  thin  mist  floating  near  the  surface 
of  the  earth ;  it  thence  increases  upwards  as  successive  layers  of 
the  air  are  cooled  below  the  point  of  saturation.  It  includes  all 
those  mists  already  described,  which  in  the  calm  evening  of  a 
warm  summer  day  form  in  the  bottom  of  valleys  and  over  low- 
lying  grounds,  and  then  spread  upwards  over  the  surrounding 
country  like  an  inundation. 

When  the  morning  sun  shines  on  the  upper  surface  of  the 
stratus  cloud,  it  begins  to  be  agitated  and  to  heave  up  in  dif- 
ferent places  into  the  rounded  forms  of  the  cumulus,  and  the 
whole  of  its  lower  surface  begins  to  rise  from  the  ground.  As 
the  heat  increases,  it  continues  to  ascend,  breaks  up  into  de- 
tached masses,  and  soon  disappears.  This  indicates  a  continu- 
ance of  fine  weather. 

27.  Cirro-cumulus. — This  cloud  is  composed  of  well-defined, 
small,  roundish  masses,  lying  near  each  other,  and  quite  separated 
by  intervals  of  sky.    It  is  formed  from  the  cirrus  cloud,  the  fibres 
of  which  break,  and  gather  into  these  small  masses.     It  is  com- 
monly known  among  sailors  as  a  mackerel  sky. 

28.  Cirro-stratus. — The  cirro-stratus  partakes  partly  of  the 
characteristics  of  the  cirrus   and   stratus.     It  consists  of  long, 
thin,  horizontal  clouds,  'with  bent  or  undulated  edges,  and  either 
separate  or  in  groups.     It  is  a  marked  precursor  of  storms. 

29.  Cumulo-stratus.  — This  cloud  is  formed  by  the  blending  of 
the   cirro-stratus  ivith   the  cumulus,  either   among  its  piled-up 
heaps,  or  spreading  underneath  its  base  as  a  horizontal  layer. 
It  is  formed  when  the  cumulus  becomes  surrounded  with  small 
fleecy  clouds  just  before  rain  begins  to  fall,  and  also  on  the 
approach  of  thunder-storms. 

30.  Cumulo-cirro-stratus,  or  Nimbtis. — This  is  the  well-known 
rain-cloud,  consisting  of  a  cloud,  or  system  of  clouds,  from  which 
rain  is  falling.      It  sometimes   has   its  origin   in  the   cumulo- 
stratus,  which  increases  till  it  overspreads  the  sky,  and  becomes 
black  or  bluish-black  in  color;  but,  this  soon  changing  to  gray, 
the  nimbus  is  formed,  and  rain  begins  to  fall. 

Its  name,  cumulo-cirro-stratus,  suggests  the  way  in  which  it 
is  more  frequently  formed.  At  a  considerable  height,  a  sheet  of 


APPENDIX. 


251 


cirro-stratus  cloud  is  spread  out,  under  which  cumulus  clouds 
drift  from  the  windward ;  these  rapidly  increase  and  unite,  form- 
ing one  continuous  gray  mass,  from  which  the  rain  falls.  The 
breaking-up  of  the  lower  gray  mass  indicates  that  the  rain  will 
soon  cease. 

s^  When  a  rain-cloud  is  seen  approaching  at  a  distance,  cirri 
appear  to  shoot  out  from  its  top  in  all  directions ;  and  the  more 
copious  the  rain-fall,  the  greater  is  the  number  of  these  cirri. 


RAIN,   SNOW,   AND   HAIL. 

31.  Rain.-^  Whatever  lowers  the  temperature  of  the  air  may 
be  considered  as  a  cause  of  rain.  It  is  chiefly  brought  about 
by  the  ascent  of  air  into  the  higher  regions  of  the  atmosphere. 
Moist  air-currents  are  forced  up  into  the  higher  parts  of  the 
atmosphere  by  colder,  drier,  and  therefore  heavier,  wind-cur- 
rents which  get  beneath  them.  Ranges  of  mountains  also 
oppose  their  masses  to  the  winds,  so  that  the  air  forced  up 
their  slopes  is  cooled,  and  its  vapor  condensed  into  showers 
of  rain  or  snow.  Moist  air-currents  are  also  drawn  up  into  the 
higher  regions  of  the  atmosphere  over  the  area  of  least  pressure 
at  the  centre  of  storms ;  and  in  such  cases  the  rain-fall  is  gen- 
erally very  heavy.  The  temperature  of  the  air  is  lowered,  and 
the  amount  of  the  rain-fall  increased,  by  those  winds  which 
convey  the  air  to  higher  latitudes.  This  occurs  in  temperate 
regions,  or  in  those  tracts  traversed  by  the  return  trade-winds, 
which  in  the  north  temperate  zone  blow  from  the  south-west, 
and  in  the  south  temperate  zone  from  the  north-west.  The 
meeting  and  mixing  of  'winds  of  different  temperatures  is  also 
v  a  cause  of  rain,  since  the  several  portions,  when  combined  into 
one,  cannot  hold  as  much  vapor  as  before.  The  rain-fall  is  also 
increased  if  the  prevailing  winds  are  directly  from  the  sea,  and 
are  therefore  moist;  but  it  is  diminished  if  they  have  passed 
over  large  tracts  of  land,  particularly  mountain-ranges,  and  are 
therefore  dry.  The  quantity  of  rain  is  influenced  by  sandy 
deserts,  which  allow  radiation,  by  day  or  night,  to  take  im- 


252  APPENDIX. 

mediate  effect  in  raising  or  depressing  the  temperature;  and 
also  by  forests,  which  retard  or  counteract  radiation. 

Rain  rarely  or  never  falls  in  certain  places,  which  are,  on 
that  account,  called  rainless  regions;  as,  for  example,  the  coast 
of  Peru  in  South  America,  the  Sahara  in  Africa,  and  the  desert 
of  Gobi,  in  Asia. 

The  Sahara  is  bounded  on  the  north  and  on  the  south  by 
ranges  of  mountains.  When  the  north-east  trade-wind  strikes 
the  northern  range,  a  part  of  its  vapor  is  condensed.  As  it 
moves  southward,  it  reaches  warmer  latitudes,  where  there  is 
a  greater  capacity  for  moisture.  Since  there  are  no  opposing 
winds  to  force  it  upwards,  it  sweeps  on  across  the  vast  sandy 
plain  until  it  arrives  at  the  southern  mountains,  where  its  vapor 
is  precipitated  in  abundant  rains.  In  the  few  spots  in  the  desert 
where  hills  or  mountains  occur,  there  are  occasional  rains. 

On  the  desert  of  Gobi,  the  prevailing  winds  are  from  the 
.south-east,  and  are  very  dry,  because  they  have  precipitated 
nearly  all  their  moisture  in  passing,  over  the  Himalaya  Moun- 
tains. 

The  rainless  district  in  Peru  is  caused  by  the  Andes,  which 
condense  nearly  all  the  vapor  of  the  south-east  trade-wind  in 
copious  rains  on  their  eastern  slopes. 

On  the  other  hand,  in  such  places  as  Chili  and  Patagonia,  it 
rains  almost  every  day.  "XT^ 

32.  Rain-fall 'within  the  Tropics. — At  places  within  the  tropics, 
where  the  trade-winds  blow  regularly  and  steadily,  the  rain-fall 
is  small.     Since  these  winds  come  from  higher  latitudes,  the 
temperature  is  increasing,  and  they  are  thus  more  likely  to  take 
up  moisture  than  to  part  with  it.    Where,  however,  the  trade- 
winds  are  forced  up  the  slopes  of  mountain  ranges,  they  bring 
rain  in  copious  showers. 

33.  The  Region  of  Calms.—  This  tropical  belt  (see  page  235) 
is  the  region  of  constant  rains.     Here  the  sun  almost  invariably 
rises  in  a  clear  sky;  but  about  midday  clouds  gather,  and  the 
whole  face  of  the  sky  is  soon  covered  with  black  clouds,  which 
pour  down  prodigious  quantities  of  rain.     Towards  evening  the 
clouds  disappear,  the  sun  sets  in  a  clear  sky,  and  the  nights 


APPENDIX.  253 

are  serene  and  fine.  The  reason  of  this  is,  that  the  air,  being 
greatly  heated  by  the  vertical  rays  of  the  sun,  ascends,  drawing 
with  it  all  the  vapor  which  the  trade-winds  have  brought  with 
them,  and  which  has  been  largely  increased  by  the  rapid  evap- 
oration from  the  belt  of  calms ;  and  this  vapor  is  condensed  as 
it  rises.  The  rain  is  sometimes  so  copious  that  fresh  water  has 
been  collected  from  the  surface  of  t£e  sea.  As  evening  sets  in, 
the  surface  of  the  earth  and  the  air  near  it  being  cooled,  the  as- 
cending currents  cease,  and  the  cooled  air  descends ;  the  clouds 
are  thus  dissolved,  and  the  sky  continues  clear  till  the  returning 
heat  of  the  following  day. 

34.  The  Rain-fall  of  India.  —  Over  a  great  part  of  the  tropics 
disturbing  influences  draw  the  trade-winds  out  of  their  course, 
and  sometimes,  as  in  the  case  of  the  monsoons,  give  rise  to 
winds  which  blow  from  the  opposite  point  of  the  compass. 
These  winds  affect  the  rain-fall  of  India,  and  but  for  them 
the  eastern  districts  of  Hindostan  would  be  constantly  deluged 
with  rain,  and  the  western  districts  constantly  dry  and  arid. 
As  it  is,  each  part  of  India  has  its  dry  and  wet  seasons,  summer 
being  the  wet  season  of  the  west  and  interior  as  far  as  the 
Himalaya,  and  winter  the  wet  season  of  the  east,  and  especially 
the  south-east. 

So  far  as  known,  the  heaviest  annual  rain-fall  at  any  place  on 
the  globe  is  600  inches,  on  the  Khasia  Hills.  About  500  inches 
of  this  fall  in  seven  months,  during  the  south-west  monsoons. 
These  hills  face  the  Bay  of  Bengal,  from  which  they  are  sepa- 
rated by  only  200  miles  of  swamps  and  marshes.  Hence  the 
southerly  winds  not  only  arrive  heavily  laden  with  vapor  from 

\Jthe  Indian  Ocean,  but  they  get  more  moisture  in  passing  over 
the  200  miles  of  swamp.  They  are,  therefore,  ready  to  burst  in 
torrents,  even  before  they  are  suddenly  raised,  by  the  hills  they 
encounter,  into  the  cooler  regions  of  the  atmosphere. 
~S  35.  Snotv.  —  Snow  is  the  frozen  moisture  which  falls  from  the 
/clouds  when  the  temperature  is  32°  or  lower.  The  particles  of 

f  >which  snow  is  composed  are  crystals,  which  are  usually  in  the 
form  of  six-pointed  stars.  About  1,000  different  kinds  of  snow- 
crystals  have  been  already  observed,  a  few  of  which  are  shown 


254  APPENDIX. 

in  Figure  199.  The  forms  of  the  crystals  of  the  same  fall  of 
snow  are  generally  similar  to  each  other.  Snow-flakes  vary 
from  an  inch  to  T^j-  of  an  inch  in  diameter,  the  largest  being 
observed  when  the  temperature  is  near  32°,  and  the  smallest  at 
very  low  temperatures. 

The  limit  of  the  fall  of  snow  at  any  time  of  the  year  coincides 
nearly  with  30°  N.  latitude,  which  includes  almost  the  whole  of 
Europe.  On  traversing  the  Atlantic  this  line  rises  to  45°,  but 
on  nearing  the  American  continent  it  descends  to  33° ;  it  rises 
in  the  west  of  America  to  47°,  and  again  falls  to  40°  in  the 
Pacific.  Snow  is  unknown  at  Gibraltar;  at  Paris,  it  falls  12 
days  on  an  average  annually,  and  at  St.  Petersburg,  170  days. 

The  white  color  of  snow  is  caused  by  the  combining  of  the 
different  prismatic  rays  which  issue  from  the  minute  snow- 
crystals.  When  the  crystals  are  looked  at  separately,  some 

Fig.  199- 


appear  red,  others  green,  purple,  and,  in  short,  all  the  colors 
of  the  spectrum;  but  when  a  mass  of  snow  is  looked  at,  the 
different  colors  blend  into  white. 

Red  snoiv  and  green  snow  have  been  occasionally  met  with  in 
the  arctic  regions  and  in  other  parts  of  the  world.  These  colors 
are  due  to  the  presence  of  vegetable  organisms,  about  y^g-^  of 
an  inch  in  diameter,  which  grow  and  flourish  in  the  region  of 
eternal  snow. 

From  its  loose  texture,  and  from  its  containing  about  ten 
times  its  bulk  of  air,  snow  is  a  very  bad  conductor  of  heat ;  and 
thus  is  an  admirable  covering  to  preserve  the  earth  from  the 
effects  of  its  own  radiation.  It  not  unfrequently  happens  in 
times  of  great  cold,  that  the  soil  is  40°  warmer  than  the  surface 


APPENDIX.  255 

of  the  snow  which  covers  it.  The  flooding  of  rivers,  from  the 
melting  of  the  snow  on  mountains  in  spring  and  summer, 
carries  fertility  into  regions  which  would  otherwise  remain 
barren  wastes. 

36.  Hail.  —  Hailstones  are  generally  of  a  conical  or  of  a 
round  shape,  and  when  cut  across  are  found  to  be  composed 
of  alternate  layers  of  clear  and  opaque  ice,  enveloping  a  white 
snowy  nucleus.  Less  frequently  they  are  composed  of  crystals 
radiating  from  the  centre  outwards.  They  vary  much  in  size, 
some  being  as  small  as  the  smallest  shot,  while  others  are  several 
inches  in  diameter.  In  August,  1813,  hailstones  the  size  of 
eggs  fell  upon  the  British  army  among  the  Pyrenees ;  the 
storm  lasted  twenty  minutes,  and  was  not  accompanied  with 
thunder  or  lightning.  June  4th,  1814,  hail,  from  13  to  15  inches 
in  diameter,  fell  in  Ohio.  In  the  Orkney  Islands,  July  24th, 
1818,  during  thunder,  a  very  remarkable  shower  of  hail  took 
place;  the  stones  were  as  large  as  a  goose's  egg,  and  mixed 
with  large  masses  of  ice. 

The  origin  of  hail  is  not  fully  understood ;  but  it  appears  to 
be  formed  by  a  cold  current  of  air  forcing  its  way  into  a  mass 
of  air  much  warmer  and  nearly  saturated,  the  temperature  of  the 
united  mass  being  below  the  freezing-point.  The  warm  moist 
air  is  easily  accounted  for,  since  hail  generally  falls  in  summer 
and  during  the  day;  but  it  is  difficult  to  account  for  the  in- 
tensely cold  current  which  is  sufficient  to  reduce  the  warm 
saturated  mass  below  32°. 

In  mountainous  regions,  cold  currents  from  the  fields  of 
snow,  rushing  down  the  sides  of  the  mountains  and  mixing 
with  the  heated  air  of  the  valleys,  are  no  doubt  frequent  causes 
of  hail;  and  such  places  are  peculiarly  subject  to  hailstorms. 

The  sudden  ascent  of  moist  warm  air  into  the  upper  regions 
of  the  atmosphere,  where  a  cold  current  prevails  at  the  time,  is, 
in  all  probability,  a  common  cause  of  hail.  This  is  confirmed 
by  the  sultry,  close  weather  which  generally  precedes  hailstorms, 
the  slight  but  sudden  fall  of  the  barometer,  the  whirlwinds  and 
ascending  currents  which  accompany  them,  and  the  fall  in  the 
temperature  which  follows  after  the  storm  has  passed. 


256  APPENDIX. 


ATMOSPHERIC  ELECTRICITY. 

j.  Electricity  in  the  Air. — The  identity  of  lightning  and 
electricity  was  first  suspected  by  Wall  in  1708,  but  it  was  re- 
served to  Franklin  to  prove  it.  In  1749,  he  suggested,  as  the 
mode  of  proof,  the  erection  of  pointed  metallic  conductors  prop- 
erly insulated.  Acting  on  this  suggestion,  Dalibard  erected 
near  Paris  a  pointed  iron  rod,  40  feet  in  length,  and  insulated; 
and,  on  the  loth  of  May,  1752,  obtained  electrical  sparks  from 
it.  In  June  of  the  same  year,  Franklin,  impatient  at  the  delay 
in  erecting  the  spire  for  his  pointed  conductor,  tried  the  experi- 
ment of  obtaining  electricity  from  the  clouds  by  flying  a  kite. 
The  kite  was  held  by  a  hempen  string,  to  the  lower  end  of 
which  a  key  was  attached ;  and  the  whole  was  insulated  by 
tying  a  silk  ribbon  to  the  key,  the  other  end  of  the  ribbon 
being  attached  to  a  post.  On  the  approach  of  the  thunder- 
cloud, he  raised  the  kite,  and  soon  the  fibres  of  the  hempen 
string  began  to  repel  each  other;  and,  at  last,  when  the  rain 
had  moistened  the  string,  he  had  the  satisfaction  of  drawing 
sparks  from  the  key. 

When  the  sky  is  cloudless,  the  electricity  is  always  positive; 
but  the  intensity  increases  with  the  height. 

When  the  sky  is  clouded,  the  electricity  is  sometimes  positive 
and  sometimes  negative,  according  to  the  electrified  condition  of 
the  clouds.  In  relation  to  the  air,  the  earth's  surface  is  always 
negative. 

The  electricity  of  the  atmosphere  is  stronger  in  winter  than 
in  summer,  increasing  from  June  to  January,  and  decreasing 
from  January  to  June.  It  is  subject  to  a  double  maximum  and 
minimum  each  day. 

38.  Sources  of  Atmospheric  Electricity.  —  (i)  Evaporation. — 
Electricity  is  produced  when  impure  water  is  evaporating,  or 
water  in  which  chemical  decomposition  is  going  on ;  none 
whatever  being  produced  by  the  evaporation  of  pure  water. 
Evaporation  from  water  containing  an  alkali  or  a  salt  gives 
off  negative  electricity  to  the  air,  and  leaves  positive  electricity 


APPENDIX. 


257 


behind ;  but  when  the  water  contains  acid,  positive  electricity  is 
given  off,  and  negative  is  left  behind.  Hence  it  is  supposed 
that  seas,  lakes,  and  rivers  are  abundant  sources  of  electricity, 
particularly  of  the  positive  sort.  (2)  Vegetation. — The  vege- 
table kingdom  is  also  a  source  of  electricity;  (a)  from  the  evap- 
oration going  on  by  which  water  is  separated  from  the  sap  of 
the  plants,  and  (£)  from  the  giving  off  of  oxygen  gas  during 
the  day,  and  carbonic  gas  during  the  night.  In  these  cases, 
positive  electricity  arises  from  the  plants,  and  negative  is  left 
behind.  (3)  Combustion.  —  During  the  process  of  burning, 
bodies  give  off  positive  electricity,  and  become  themselves  neg- 
atively electrified.  This  is  frequently  seen  on  a  grand  scale 
during  volcanic  eruptions.  (4)  Friction. — Wind,  by  the  fric- 
tion it  produces  upon  terrestrial  objects,  the  particles  of  dust, 
and  the  watery  particles  which  it  carries  with  it,  contributes  to 
the  electricity  of  the  air.  Electricity  is  not  generated  if  the 
moisture  be  in  the  form  of  pure  vapor. 

39.  Effect  of  the  Condensation  of  Vapor.  — When  a  great  mul- 
titude of  molecules  of  vapor  are  condensed  by  cold  into  a  drop, 
or  snow-spangle,  that  drop  probably  collects  and  retains  on  its 
surface  the  whole  electricity  of  the  molecules  from  which  it  is 
formed.     If  a  thousand   such   globules  coalesce   into   one,  the 
electricity  will  be  increased  a  thousand-fold,  and,  being  spread 
entirely  over  the   surface,  will  have  a  tenfold  tension.     This 
view  (which  is  Sir  John  Herschel's)  explains  the  electricity  ob- 
served in  the  lower  stratum  of  air  when  detv  is  being  deposited, 
and  the  highly  electrical  state  of  fogs  and  clouds.     It  also  ex- 
plains the  annual  fluctuation ;  for,  since  in  winter  the  conden- 
sation of  vapor  is  greater  and  more  frequent  than  in  summer, 
the  average  quantity  of  electricity  will  be  greater  in  winter. 

40.  Thunder-storms. — The  thunder-storm  probably  originates, 
like  cloud  and  rain,  in  the  condensation  of  vapor ;  but  the  con- 
densation is  more  copious  and  more  rapid,  so  as  to  bring  about 
an  accumulation  of  a  sufficient  quantity  of  electricity.     If  the 
condensation  is  not  copious,  the  electricity  will  be  too  weak; 
and  if  not  sudden,  it  escapes  before  enough  collects  for  a  dis- 
charge. 

17 


258  APPENDIX. 

Thunder-storms  occur  most  frequently  within  the  tropics,  and 
diminish  in  frequency  towards  the  poles.  They  are  also  more 
frequent  in  summer  than  in  winter;  during  day  than  during 
night ;  after  midday  than  before  it ;  and  in  mountainous  coun- 
tries than  in  plains.  Within  the  tropics  they  prevail  most  in 
the  region  of  calms  and  during  the  rainy  season ;  and  least 
in  arid  deserts  and  during  the  dry  season. 

41.  Lightning.  —  Arago    has    divided    lightning    into    three 
kinds;    zigzag  lightning,   sheet   lightning,    and   ball  lightning. 
When   the   electric  flash    darts    through   the   air,    it  takes   the 
path  of  least  resistance ;   and,   since  the  conducting  power  of 
different  portions  of  the  atmosphere  is  unequal,  the  lightning 
frequently  appears  zigzag.     When   branches   are   given   off  at 
different  points  of  its  course,  the  lightning  is  said  to  be  forked. 
Sheet  lightning  is  the  most  common,  appearing  as  a  glow  of 
light  illuminating  the  sky.    The  flashes  often  follow  each  other 
in  quick  succession,  and  the  thunder  which  accompanies  them 
is  low  and  at  a  considerable  distance.     Analogous  to  this  is 
silent  lightning,   often   called   heat  lightning,  which   generally 
occurs   during  serene   summer  evenings,  lighting  up   the  sky 
for  hours  with  repeated  faint  flashes,  attended  with  no  thunder. 
It  is  probable  that  this  kind  of  lightning  is  almost  always  the 
reflection  of  the  lightning  of  distant  storms  from  the  vapor  of 
the   upper  regions  of  the   atmosphere.     Ball  lightning  is   the 
least  common.     It  appears  as  a  globular  mass,  moving  slowly 
or  sometimes  remaining  stationary,  and- in  a  short  time  explodes 
with  violence.    It  has  not  yet  been  satisfactorily  explained.    The 
duration  of  a  flash  of  lightning,  like  that  of  an  electric  spark,  is 
less  than  the  thousandth  part  of  a  second.     For  this  reason 
a  wheel  rotating  so  fast  that  the  spokes  are  invisible  will  ap- 
pear stationary  when  lighted  up  with  the  electric  spark  or  by 
lightning. 

42.  Thunder.  —  Thunder  is  probably  the  noise  produced  by  the 
rush  of  the  air  to  fill  the  vacuum  left  by  the  lightning  along  the 
path  of  the  discharge.    The  sound  emitted  by  flames  is  a  familiar 
phenomenon  of  the  same  kind.     Flashes  of  lightning  frequently 
extend  two  or  three  miles  in  length ;  and  since  the  thunder  is 


APPENDIX.  259 

produced  along  its  whole  course  nearly  at  the  same  instant,  the 
prolonged  rolling  noise  arises  from  the  different  intervals  of 
time  it  takes  the  sound  to  reach  the  ear.  Reverberations  from 
clouds  and  from  mountains  frequently  heighten  the  effect  and 
prolong  the  peal.  Thunder  has  not  been  heard  at  a  greater 
distance  than  14  miles  from  the  flash. 

43.  Effects   of  Lightning.  —  Electrical    discharges    generally 
pass  into  the  air,  or  into  other  clouds  less  highly  electrified ; 
a  very  few  only  take  place  between  the  cloud  and  the  earth. 
By  this  latter  class  innumerable  lives  have  been  destroyed,  the 
strongest  trees  rent  to  pieces,  heavy  bodies  displaced,  iron  and 
steel  magnetized,  metals  and  rocks  softened  and  fused,  and  com- 
bustible substances  set  on  fire.    When  the  thunderbolt  falls  upon 
sand,  it  usually  produces  fulgurites,  or  fulminary  tubes,  which 
are  silicious  tubes  of  various  sizes  vitrified  within. 

44.  Return  Shock.  —  The  return  shock  sometimes  proves  fatal 
to  living  beings,  even  at  great  distances  from  the  place  of  the 
electric  discharge.     It  is  caused  by  the  inductive  action  of  the 
electrified  cloud,  by  which  bodies  become  charged  with  the  elec- 
tricity opposite  to  that  of  the  cloud.     When  the  cloud  has  dis- 
charged its  electricity  into  the  ground,  the  induction  ceases,  and 
the  rapid  change  of  the  bodies  from  the  electrified  to  the  neutral 
state  gives  them  a  severe  shock. 

45.  Lightning-Rods.  —  The    lightning-rod   was    invented    by 
Franklin   in   1755.     The   chief  advantage   gained   by  it  is  not 
that  it  protects  the  building  in  case  of  a  discharge  by  allow- 
ing a  free  passage  for  the  electric  fluid  to  the  earth ;  but  by 
quietly  and  gradually  keeping  up  the  communication,  it  tends 
to  maintain  the  electric  equilibrium,  and  thus. prevent  a  dis- 
charge.    The  best  rods  are  made  of  copper,  not  less  than  three- 
quarters  of  an  inch  thick,  and  pointed  at  the  upper  end.     They 
should  be  of  one  piece  throughout,  fastened  vertically  to  the 
roof  of  the  building,  and  thence  carried  down  into  the  ground. 
The  lower  extremity  should  part   into  two  or  three  branches 
bent  away  from  the  house,  and  carried  far  enough  into  the  soil 
to   meet  water,  or  permanently  moist  earth.     The   conductor 
should  be  connected  with  all  metallic  surfaces  on  the  roof  or 


26o  .  APPENDIX. 

other  parts  of  the  building,  in  order  to  prevent  the  occurrence 
of  lateral  discharges,  or  discharges  from  the  conductor  to  these 
surfaces,  which  are  often  very  destructive.  ^V~ 

46.  St.  Elmo's  Fire. — This  meteor  is  the  Castor  and  Pollux 
of  the  ancients,  and  is  frequently  mentioned  in  classic  writings. 
The   finest   and    most  beautiful    displays   occur   at   sea   during 
storms,  when  it  appears  as  a  light  resting  on  the  masts.     The 
light  which  is  seen  on  a  point  held  near  the  conductor  of  an 
electric  machine  explains  St.  Elmo's  fire.     It  takes  place  when 
the  electricity  of  a  cloud  and  that  of  the  earth  combine,  not  in 
flashes  of  lightning,  but  slo-wly  and  continuously  from  different 
points. 

47.  The  Aurora  Borealis.  —  The  aurora  borealis  %is  a  luminous 
appearance  in  the   northern  sky.     It  is  observed   also  in   the 
neighborhood   of  the   south   pole,   and  is  there   called   aurora 
australis.     When  fully  developed,  the  aurora  consists  of  a  dark 
segment  of  a  hazy  or  slaty  appearance,  surmounted  by  an  arch 
of  light,  from  which   luminous  streamers  quiver  and  dart  up- 
wards.     Several  auroral  arches   are   sometimes    seen   at  once. 

X\  Sometimes  the  streamers  appear  to  unite  near  the  zenith,  form- 
ing what  is  called  the  corona  of  the  aurora,  towards  which  the 
dipping  needle  at  the  time  points. 

Auroras  are  very  unequally  distributed  over  the  earth's  sur- 
face. At  Havana,  but  six  have  been  recorded  within  a  hundred 
years.  As  we  travel  northwards  from  Cuba,  they  increase  in 
frequency  and  brilliancy;  they  rise  higher  in  the  heavens,  and 
oftener  attain  the  zenith.  If  we  travel  northwards  along  the 
meridian  of  Washington,  we  find,  on  an  average,  near  the  par- 
allel of  40°,  only  ten  auroras  annually.  Near  the  parallel  of 
42°,  the  average  number  is  twenty  annually;  near  45°,  it  is 
forty;  and,  near  50°,  it  is  eighty.  Between  this  point  and  the 
parallel  of  62°,  auroras  are  seen  almost  every  night,  high  in 
the  heavens,  and  as  often  to  the  south  as  the  north.  Farther 
north,  they  are  seldom  seen  except  in  the  south,  and  from  this 
point  they  diminish  in  frequency  and  brilliancy  as  we  advance 
towards  the  pole.  If  we  make  a  like  comparison  for  the  me- 
ridian of  St.  Petersburg,  we  shall  find  a  similar  result,  except 


APPENDIX.  26l 

that  the  auroral  region  is  situated  farther  northward  than  it  is 
in  America.  Auroras  are  more  frequent  in  the  United  States 
than  they  are  in  the  same  latitudes  of  Europe. 

The  aurora  is  of  great  extent,  having  been  sometimes  ob- 
served simultaneously  in  Europe  and  America.  The  height 
varies  from  about  45  to  500  miles  above  the  earth. 

48.  Relations  of  the  Aurora  to  Magnetism.  — Many  facts  show 
a  connection   between   the   aurora  and   terrestrial   magnetism. 
The  magnetic  needle  is  much  agitated  when  the  aurora  is  vis- 
ible.    When  the  arch  is  motionless,  so  is  the  needle;  but  as 
soon  as  streamers  are  shot  out,  its  declination  changes  every 
moment,  and  this  happens  though  the  aurora  does  not  appear 

,  at  the  place  of  observation,  but  is  seen  near  the  pole.  It  is 
v  probable  that  magnetic  disturbances  of  the  earth  are  due  to 
the  sun,  but  not  to  his  heat  and  light;  and  that  they  are 
invariably  accompanied  by  the  aurora  and  by  electric  currents 
on  the  surface  of  the  earth.  The  secular  periods  of  the  sun's 
spots,  of  the  variation  of  the  magnetic  needle,  and  of  the  fre- 
quency of  auroras,  coincide  in  a  remarkable  way,  indicating 
1;hat  these  phenomena  are  regulated  by  astronomical  causes.  * 

OPTICAL  PHENOMENA. 

49.  The  Rainbow.  —  The  rainbow,  in  its  most  perfect  form, 
consists  of  two  colored  arches  projected  upon  falling  rain  on 
which   the   sun   is   shining   from   the   opposite   quarter  of  the 
heavens.     The  lower  or  inner  arch  is  called  the  primary  bow; 
the  upper  or  outer,  the  secondary  bow.     Each  contains  all  the 
colors  of  the  spectrum,  but  the  order  of  the  colors  in  one  is 
the  reverse  of  that  in  the  other.     Red  is  outermost  in  the  pri- 
mary bow,  and  innermost  in  the  secondary.     The  primary  bow 
is  the   narrower   and  brighter  of  the   two,   and  when  it  is  of 
unusual  brightness  narrow  red  arches  are  seen  just  within  it, 
called  supernumerary  bows.     These  are  sometimes  three  or  four 
in  number,  but  they  can  be  traced  only  a  short  distance.    The 

*  See  Handbook  of  the  Stars,  p.  91. 


262  APPENDIX. 

common  centre  of  the  bows  is  in  a  line  drawn  from  the  sun 
through  the  eye  of  the  observer. 

The  rainbow  is  produced  by  the  refraction  and  reflection  of 
the  sunlight  •within  the  rain-drops.  Its  colors  are  due  partially 
to  the  dispersion,  and  partially  to  the  interference  of  the  light 
thus  refracted  and  reflected. 

Rainbows  in  the  morning  are  always  seen  in  the  west,  and 
indicate  the  advance  of  the  rain-cloud  from  the  west  at  the  time 
that  it  is  clear  and  bright  in  the  east.  Since  the  fall  of  rain  at 
this  time  of  the  day  when  the  temperature  should  be  rising  is 
an  additional  evidence  of  increasing  moisture,  a  morning  rain- 
bow is  a  prognostic  of  a  change  to  wet,  stormy  weather.  On 
the  contrary,  a  rainbow  in  the  evening  shows  the  passing  of  the 
rain-cloud  to  the  east,  and  a  clearing  up  in  the  west  at  the  time 
of  day  when  the  temperature  has  begun  to  fall ;  thus  further 
indicating  a  change  from  wet  to  dry  weather.  Hence  the  popu- 
lar rhyme :  — 

"  A  rainbow  in  the  morning,  — 
Sailors  take  warning ; 

A  rainbow  at  night  • 

Is  the  sailor's  delight." 

50.  Lunar  Rainbows.  —  Rainbows  are  also  produced  by  the 
light  of  the  moon  falling  on  rain-drops,  exactly  in  the  same 
way  as  solar  rainbows.  They  are  by  no  means  of  rare  occur- 
rence. Owing  to  the  feeble  light  of  the  moon,  the  bow  is 
generally  without  colors ;  but  when  the  sky  is  very  clear  and 
the  moon  is  full,  the  prismatic  colors  appear,  though  in  sub- 
dued splendor. 

51.  Coronas.  —  The  corona  is  an  appearance  of  faintly  colored 
rings  encircling  the  moon  when  seen  behind  the  light,  fleecy 
cloud  of  the  cirro-cumulus.  When  the  corona  is  perfect,  the 
rings  form  several  concentric  circles,  the  blue  prismatic  color 
being  nearer  the  centre  than  the  red.  When  large,  the  ring  has 
generally  a  whitish,  nebulous  appearance. 

Coronas  are  also  very  frequently  formed  round  the  sun ;  but 
to  see  them  it  is  necessary  to  look  through  smoked  glass,  or  at 
the  image  of  the-sun  reflected  from  still  water. 


APPENDIX.  263 

52.  Anthelia.  —  Glories  of  light,   otherwise   called   anthelia, 
because  formed  opposite  the  sun,  are  sometimes  seen  when  the 
shadow  of  an  observer  is  cast  on  fog;   and  the  shadow  of  his 
Fig.  200.  Fig.  201. 


Fig.  202. 


Fig.  203. 


Fig.  204. 


Fig.  205. 


head  is  surrounded  with  the  prismatic  circles.  The  phenomenon 
is  seen  in  the  polar  regions  whenever  sunshine  and  fog  occur  at 
the  same  time. 


264  APPENDIX. 

53.  Halos.  —  Halos  are  circles  of  prismatic  colors  around  the 
sun  (Figures  200-203),  or  the  moon  (Figures  204  and  205),  but 
they  are  not  to  be  confounded  with  coronas.  Halos  are  of 
comparatively  rare  occurrence;  coronas,  on  the  contrary,  may 
be  seen  every  time  a  light,  fleecy  cloud  comes  between  us  and 
the  sun  or  moon.  The  structure  of  halos,  as  seen  from  the 
figures,  is  often  very  complicated,  circle  cutting  circle  with 
mathematical  exactness,  the  circles  being  generally  very  large. 
The  structure  of  the  corona,  on  the  other  hand,  is  simple,  the 
circles  concentric,  the  inner  one  small,  the  diameter  of  the 
second  being  double,  and  that  of  the  third  treble,  the  diameter 
of  the  first. N  In  halos,  the  red  prismatic  color  is  next  the  centre; 
in  coronas,  the  blue.  Halos  are  formed  by  the  refraction  and 
reflection  of  the  rays  of  light  by  the  minute  snow-crystals  of  the 
cirrus  cloud ;  while  coronas  arise  from  the  interference  of  the 
rays  passing  on  each  side  of  the  globules  of  vapor. 

At  the  points  where  the  circles  of  the  halo  intersect,  images  of 
the  sun  or  moon  generally  appear,  from  the  light  concentrated 
at  these  points.  The  images  of  the  sun  are  called  parhelia,  or 
mock- suns  ;  and  those  of  the  moon,  paraselene,  or  mock-moons. 
These  also  exhibit  the  prismatic  colors  of  the  halo. 
*•  54.  Colors  of  Clouds.  — The  red  and  golden  clouds  which  fire 
the  western  sky  at  sunset,  are  the  accompaniment  of  cumulus 
clouds  as  they  slowly  sink,  while  dissolving,  down  into  the 
lower  and  warmer  parts  of  the  atmosphere;  and  consequently 
they  disappear  from  the  sky  shortly  after  sunset.  Such  sunsets 
are  therefore  prophetic  of  fine  'weather. 

A  green  or  yellowish-green  sky,  on  the  other  hand,  is  one  of 
the  surest  prognostics  of  rain  in  summer,  and  snow  in  winter. 
If,  after  a  storm,  the  yellow  tint  becomes  of  a  sickly  green,  more 
rain  may  be  expected ;  but  if  it  deepens  into  orange  and  red,  the 
atmosphere  is  getting  drier,  and  fine  weather  may  be  looked  for. 

It  has  been  shown  that  high-pressure  steam,  while  trans- 
parent, and  in  the  act  of  expansion,  readily  absorbs  the  violet, 
blue,  and  part  of  the  green  rays,  thus  letting  the  yellow,  orange, 
and  red  pass.  It  is  found,  also,  that  successive  layers  of 
air,  with  visible  vapor  diffused  through  them,  separate  the 


APPENDIX.  265 

transmitted  light  more  and  more  perfectly  from  its  more  re- 
frangible rays.  The  blue  rays  are  first  absorbed,  then  the  yellow 
rays,  and  finally  the  red  rays.  It  is  in  the  lower  layers  of  the 
atmosphere  that  dust,  smoke,  watery  vapor,  and  small  rain- 
drops are  chiefly  suspended.  When  the  sun  is  high  in  the 
heavens,  the  thickness  of  the  vapor-screen  between  the  sun 
and  the  eye  has  no  perceptible  action  on  the  rays  of  light, 
which  consequently  appear  white ;  but  as  the  sun  descends  to 
the  horizon,  the  thickness  of  the  vapor  is  greatly  increased, 
and,  at  sunset,  the  light  of  the  sun  has  to  pass  through  200 
miles  of  the  air  in  illuminating  a  cloud  a  mile  above  the  earth. 
Hence,  as  the  rays  fall  more  and  more  obliquely  on  the  clouds, 
they  appear  successively  yellow,  orange,  and  finally  red.  The 
varied  colors  often  seen  at  sunset  are  due  to  the  fact  that  the 
clouds  appear  at  different  heights  and  in  different  parts  of 
the  sky,  so  that  various  thicknesses  of  vapor  are  interposed 
between  them  and  the  sun.  At  dawn,  the  clouds  first  appear 
red ;  but,  as  the  sun  rises  higher,  the  yellow  light  ceases  to  be 
absorbed,  and  they  appear  orange,  yellow,  and  finally  white. 
These  changes  are  well  described  in  Dante's  Purgatorio:  — 

The  dawn  was  vanquishing  the  matin  hour, 

Which  fled  before  it,  so  that  from  afar 

I  recognized  the  trembling  of  the  sea.  .  .  . 
Already  had  the  sun  the  horizon  reached,  .  .  . 
So  that  the  white  and  the  vermilion  cheeks 

Of  beautiful  Aurora,  where  I  was, 

By  too  great  age  were  changing  into  orange. 

LongJ feltou?s  Translation. 

A  high  red  dawn  is  a  prognostic  of  settled  weather,  because 
the  redness  seen  in  clouds  at  a  great  height  while  the  sun  is  yet 
below  the  horizon,  may  be  occasioned  by  the  great  thickness - 
of  the  vapor-screen  through  which  the  rays  must  pass  before 
reaching  the  Clouds,  and  not  by  any  excess  of  vapor  in  the  air 
itself.  But  if  the  clouds  be  red  and  lowering  in  the  morning,  it 
is  a  sign  of  rain ;  since,  the  thickness  traversed  by  the  rays 
being  now  much  less,  the  red  color  must  arise  from  an  unusual 
amount  of  vapor  in  that  stage  of  partial  condensation,  when  the 
blue  rays  are  absorbed,  and  the  yellow  and  red  pass. 


266  APPENDIX. 


MOLECULAR  MOTION  AS    MANIFESTED   IN  SOUND, 
LIGHT,    HEAT,   AND   ELECTRICITY. 

i.  Sound-waves. — When  water  is  agitated,  the  molecules 
vibrate  in  sets,  one  set  moving  upward  while  the  next  set  is 
moving  downward,  thus  giving  rise  to  a  wave. 

When  two  sets  of  water-waves  meet  and  cross,  they  are  found 
at  certain  points  to  increase,  and,  at  others,  to  diminish  each 
other's  volume.  The  former  takes  place  when  they  meet  in  the 
same  phase, —  that  is,  when  the  hollow  of  one  meets  the  hollow 
of  the  other,  or  the  crest  of  one  meets  the  crest  of  the  other ; 
while  the  latter  occurs  when  they  meet  in  opposite  phases,  — 
that  is,  when  the  crest  of  one  meets  the  hollow  of  the  other. 
If  the  waves  are  of  the  same  size,  they  will  in  the  one  case 
destroy  each  other,  and,  in  the  other  case,  form  a  wave  of 
double  the  height. 

Now  we  have  seen  that  two  sounds  may  meet  so  as  to  destroy 
each  other,  or,  as  in  the  case  of  beats,  so  as  alternately  to  in- 
crease and  diminish  each  other.  This  must  be  because,  in 
sonorous  vibrations,  the1  molecules  vibrate  in  sets,  so  as  to 
produce  waves.  When  a  string,  for  instance,  vibrates,  the  air 
about  it  is  alternately  compressed  and  extended ;  and  these 
compressions  and  extensions  run  on  in  the  direction  in  which 
the  sound  travels,  and  constitute  sound-waves.  The  molecules 
vibrate  longitudinally,  —  that  is,  backward  and  forward ;  and 
not,  as  in  the  water-wave,  transversely,  —  that  is,  at  right  angles 
to  the  direction  in  which  the  wave  moves. 

Sound-waves  interfere  so  as  to  destroy  each  other  when  they 
fcmeet  in  opposite  phases ;  that  is,  when  the  compression  of  the 
one  meets  the  extension  of  the  other. 

As  the  pitch  of  the  sound  rises,  the  vibrations  become  more 
rapid  and  the  waves  shorter;  for  the  length  of  a  sound-wave  is 
the  distance  that  the  sound  travels  while  the  sounding  body 
is  making  a  single  vibration. 

Sounds  give  beats  when  they  differ  slightly  in  pitch.  The 
waves  which  then  flow  out  from  the  sounding  bodies  differ 


APPENDIX.  267 

slightly  in  length,  and  encounter  each  other  alternately  in  the 
same  and  opposite  phases. 

2.  Reflection  of  Sound. — The  transmission  of  sound  through 
air,  or  any  other  elastic  medium,  is  best  illustrated  by  a  row  of 
ivory  balls.     If  the  balls  are  all  of  the  same  size,  each  gives  up 
all  its  motion  to  the  next,  and  itself  comes  to  rest.     If  one  of 
the  balls  is  larger  than  the  next,  it  gives  up  only  a  part  of  its 
motion  to  it.     If  it  is  smaller  than  the  next,  it  puts  that  in  mo- 
tion, and  itself  rebounds.     When  sound  is  travelling  through 
the  same  medium,  we  have  the  condition  of  the  balls  of  the 
same  size.     Each  molecule  gives  up  all  its  motion  to  the  next, 
and  itself  comes  to  rest.     When  the  sound  meets  a  denser  me- 
dium, we  have  the  condition  of  a  smaller  ball  striking  against 
a  larger  one.     The   molecules  of  the   denser  medium  are  set 
vibrating,  while  those  of  the  rarer  medium  rebound  and  trans- 
mit their  motion  backward,  so  that  a  part  of  the  sound-wave 
is  reflected.     When  the  sound-wave  meets  a  rarer  medium,  we 
have  the  condition  of  a  larger  ball  striking  a  smaller  one.     The 
molecules  forming  the  last  layer  of  the  denser  medium  retain  a 
part  of  their  motion,  and  transmit  it  back  again  to  the  mole- 
cules behind.     In  this  case,  also,  the  wave  is  partially  reflected. 

Hence,  whenever  a  sound-wave  meets  a  medium  different  in 
density  from  that  in  which  it  has  been  travelling,  it  is  partially 
reflected  and  partially  transmitted. 

3.  Refraction  of  Sound.  —  So  long  as  sound  is  traversing  the 
medium  in  which  it  originates,  the  advancing  wave  will  have  a 
spherical  outline,  since  the  sound  travels  with  equal  speed  in  all 
directions.     But  when  the  wave  passes  into  a  medium  in  which 
it  travels  at  a  different  rate,  its  outline  is  changed.     If  it  travels 
faster  in  the  new  medium,  the  portion  of  the  wave  in  it  will  be 
rounded  out,  or  become  more  convex;   if  it  travels  slower,  it 
will  be  flattened,    or  become   less   convex.     The   direction   in 
which  any  portion  of  a  sound-wave  is  travelling  will  be  repre- 
sented by  a  line  drawn   perpendicular  to  the   surface  of  that 
portion.     Let  a  b  (Figure  206)  be  a  portion  of  a  sound-wave 
moving  in  the  direction  of  the  arrow,  and  a  c  be  the  surface  of 
a  medium  O  of  different  density  from  M,  in  which  the  wave  has 


268 


APPENDIX. 


been  moving.  If  the  elasticity  of  O  is  such  that  the  wave  will 
move  faster  in  it  than  in  M,  the  portion  a  of  the  wave  which 
enters  O  first  will  move  on  faster  than  the  portion  b  while  the 

Fig.  206.  Fig.  207. 


latter  is  moving  in  M.  When  a  b  is  wholly  within  0,  the  second 
arrow  shows  the  direction  in  which  it  will  be  moving;  and  it 
will  continue  to  move  in  this  direction  so  long  as  it  is  wholly  in 
this  medium.  When  the  direction  of  a  wave  is  thus  bent,  it  is 
said  to  be  refracted.  In  this  case,  it  is  bent  away  from  a  perpen- 
dicular P  <^  drawn  to  the  surface  of  the  medium  O. 

If  the  elasticity  of  O  is  such  that  the  sound-wave  moves 
slower  in  it  than  in  M,  the  portion  a  of  the  wave  (Figure  207), 
when  it  has  entered  O,  moves  slower  than  b  while  the  latter  is 
in  M.  In  this  case,  it  will  be  seen  that  the  direction  of  the  wave 
will  be  bent  towards  the  perpendicular  P  «^. 

It  is  evident  that,  if  a  b  had  not  met  the  medium  O  obliquely, 
both  ends  of  it  would  have  entered  O  at  the  same  time,  and  its 
direction  would  not  have  been  changed. 

We  see,  then,  that  when  a  sound-wave  passes  obliquely  into 
a  medium  of  different  density,  it  is  refracted,  and  that,  if  it 
travels  more  rapidly  in  the  new  medium,  it  will  be  bent  away 
from  a  perpendicular  drawn  to  the  surface  of  that  medium ; 
while,  if  it  travels  less  rapidly  in  the  new  medium,  it  will 
be  bent  towards  a  perpendicular  drawn  to  its  surface. 

This  refraction  of  a  sound-wave  has  been  shown  by  the  ex- 


APPENDIX. 


269 


periment  illustrated  in  Figure  208.     B  is  a  collodion  balloon 

filled  with  carbonic  acid  gas;  w  is  a  watch  hung  near  it;  and 

i  is  a  glass  funnel.     By  placing  the  ear  at  /,  and  moving  the 

Fig.  208. 


Fig 


funnel  about,  a  point  will  be  found  where  the  ticking  of  the 
watch  will  be  louder  than  elsewhere.  This  shows  that  the  sound- 
waves have  been  converged  to  that  point. 

Figure  209  shows  how  the  sound-waves  are  refracted  in 
passing  through  the  carbonic 
acid,  a  b  is  a  portion  of  the 
sound-wave.  In  passing  into 
the  carbonic  acid,  —  a  medium 
in  which  it  moves  more  slowly 
than  in  air,  —  it  is  bent  into 
the  form  of  m  o!  n.  On  passing 
out  from  the  carbonic  acid,  it  is 
bent  still  farther  in  the  same  di- 
rection, and  thus  the  two  parts  of 
the  wave  are  made  to  converge. 

4.  Light  is  propagated  by  Vibrations  and  Waves.  —  We  have 
seen  that  Newton's  rings  are  produced  by  the  interference  of 
rays  of  light.  This  interference  of  light  leads  us  to  the  conclu- 
sion that  light  is  also  propagated  by  waves,  which  augment  or 
destroy  each  other,  according  as  they  meet  in  the  same  or 
opposite  phases.  These  waves,  of  course,  like  those  of  water 
and  sound,  must  be  made  up  of  vibrating  molecules.  But  light 
will  traverse  a  vacuum  as  well  as  the  air.  The  medium,  then, 
which  transmits  it,  must  exist  in  a  vacuum  and  among  the 


270  APPENDIX. 

molecules  of  all  transparent  substances.  This  medium  is  called 
the  luminiferous  ether,  or  simply  the  ether.  The  existence  of 
luminous  waves  is  the  only  evidence  of  the  existence  of  the 
ether,  but  this  evidence  is  of  such  a  kind  that  scientific  men 
generally  deem  it  conclusive.  The  ether  fills  not  only  the 
spaces  between  the  earth  and  the  sun  and  stars,  but  the  spaces 
between  the  molecules  of  all  bodies. 

Since  light,  like  sound,  is  propagated  by  vibrations,  it  is 
probable  that  it  originates  in  vibrations.  Moreover,  in  ordinary 
combustion,  which  is  the  most  familiar  source  of  light,  the 
atoms  of  the  oxygen  in  the  air  are  rushing  into  combination 
with  the  atoms  of  the  burning  body;  and  the  collision  of  these 
atoms  will  be  very  likely  to  set  them  vibrating. 

The  vibrating  molecules  first  communicate  their  vibrations  to 
the  molecules  of  the  ether  about  them,  and  these  transmit  the 
vibrations  with  the  enormous  velocity  of  190,000  miles  a  second. 

When  the  vibrations  meet  a  new  medium,  a  part  of  them 
may  pass  on  through  the  spaces  among  its  molecules  without 
disturbance,  and  thus  some  of  the  light  is  transmitted.  A  part 
of  the  vibrations  may  rebound,  and  thus  some  of  the  light  is 
reflected.  Another  part  may  be  taken  up  by  the  molecules  of  the 
medium,  and  thus  some  of  the  light  is  absorbed. 

5.  The  Length  of  the  Luminous  Wave.  —  As  the  length  of 
the  sound-wave  is  the  distance  which  sound  travels  while  the 
sounding  body  is  vibrating  once,  so  the  length  of  the  luminous 
wave  is  the  distance  which  light  travels  while  the  molecules  of 
the  luminous  body  are  making  one  vibration.  Of  course  the 
quicker  the  vibrations,  the  shorter  the  waves. 

The  length  of  the  luminous  waves  can  be  found  by  means  of 
Ne-wtorfs  rings.  If  the  curved  glass  (Figure  210)  be  pressed  down 
upon  the  plate  beneath,  and  perpendicular  rays  of  red  light  be 
allowed  to  fall  upon  it,  the  centre  a  will  be  black,  and  black 
rings  will  appear  at  i,  2,  3,  and  4.  Since  the  centre  is  black, 
the  waves  reflected  from  the  two  surfaces  must  meet  there  in 
opposite  phases,  although  the  two  surfaces  are  in  contact. 
This  is  because  a  wave  of  light  when  reflected  from  a  rarer 
medium  changes  its  phase,  while  it  does  not  when  reflected  from 


APPENDIX. 


271 


a  denser  medium.  The 
light  reflected  from  b 
must  travel  a  wave- 
length farther  than  that 
reflected  from  i,  in  order 
that  the  waves  reflected 
from  these  points  may 
meet  in  opposite  phases, 
and  so  give  a  dark  ring. 
Now  the  wave  reflected 
from  b  must  evidently 
travel  over  the  space  i  b 
twice :  hence  i  b  must 
be  i  the  length  of  a 
luminous  wave.  For  a 
similar  reason,  2  c  is  a  a  b  .  c  d  e 

wave-length ;  3  d ,  a  wave-length  and  a  half;  4  e,  two  wave- 
lengths. Now  we  can  easily  find  the  length  of  4  e.  4  m  is  half 
the  diameter  of  the  fourth  dark  ring,  and  is  found  by  measure- 
ment. We  can  also  measure  the  radius  4  C,  and  4  m  C  is  a 
right-angled  triangle.  In  this  triangle,  we  know  the  length  of 
the  hypothenuse  4  C,  and  of  the  side  4  m ;  hence  we  can  find 
C  m.  The  radius  C  a — Cm  =  a  #2  =  4  e.  If  violet  light  be 
used,  the  fourth  ring  is  smaller,  and  4  e  is  shorter.  In  this 
way,  the  wave-lengths  in  the  following  Table  are  found:  — 


Colors. 

Length  of 
waves  in  parts 
of  an  inch. 

Number  of 
waves  in  an 
inch. 

Number  of  waves  in  a 
second. 

Extreme  Red 

.0000266 

37><HO 

458,000,000,000,000 

Red 

.0000256 

39,180 

477,000,000,000,000 

Orange 

.0000240 

41,610 

506,000,000,000,000 

Yellow 

.0000227 

44,000 

535,000,000,000,000 

Green 

.OOOO2  1  1 

47,460 

577,000,000,000,000 

Blue 

.0000196 

51,110 

622,000,000,000,000 

Indigo 

.0000185 

54*070 

658,000,000,000,000 

Violet 
Extreme  Violet 

.0000174 
.0000167 

57*490 
59*750 

699,000,000,000,000 
727,000,000,000,000 

272  APPENDIX. 

According  to  Eisenlohr,  the  length  of  the  waves  in  the  extreme 
red  raj  is  just  double  the  length  of  the  waves  in  the  invisible 
rays  beyond  the  violet.  The  whole  range  of  rays,  then,  extends 
only  over  what  is  equivalent  to  a  single  octave  in  music. 

The  numbers  in  the  last  column  of  the  above  Table  show 
the  rate  at  which  the  -  molecules  of  a  body  must  vibrate,  in 
order  to  produce  the  different  colors. 

The  molecules  of  certain  substances  seem  to  be  capable  of 
vibrating  in  all  periods,  and  thus  of  producing  white  light; 
while  those  of  other  substances  seem  to  be  capable  of  vibrating 
only  in  particular  periods,  and  therefore  they  produce  light  of 
different  colors.  It  is  seldom,  however,  that  the  vibrations 
of  the  molecules  are  limited  to  one  period,  and  therefore  that  a 
luminous  body  gives  out  homogeneous  light.  We  can  now 
understand  how  it  is  that  we  can  detect  certain  substances  by 
the  light  they  give.*  Their  particles  can  vibrate  only  in  certain 
ways,  and  they  of  course  cause  the  particles  of  ether  nearest 
them  to  vibrate  in  the  same  way.  The  vibrations  are  sent  on 
unchanged  from  particle  to  particle  of  the  ether,  and  are  ready 
at  any  point  to  reveal  the  nature  of  the  substance  in  which  they 
originated.  The  vibrations  are  so  minute  ^that  it  would  seem 
impossible  to  find  out  their  character,  but  the  spectroscope 
enables  us  to  do  this  with  ease  and  accuracy. 

When  a  number  of  strings  of  different  lengths  and  tension 
are  stretched  in  the  air,  as  in  the  ./Eolian  harp,  they  absorb  all 
the  vibrations  accordant  to  their  own  which  fall  upon  them, 
while  they  allow  all  the  discordant  ones  to  pass  on.  In  much 
the  same  way,  we  must  imagine  the  molecules  of  a  body  sus- 
pended in  the  ether,  from  which  they  absorb  all  accordant  vibra- 
tions while  they  transmit  all  discordant  ones. 

Transparency  is  then  synonymous  with  discordance,  and 
opacity  with  accordance.  This  explains  the  fact  that  different 
substances  absorb  light  of  different  vcolors,  and  also  the  fact 
that  incandescent  gases  give  out  light  of  the  same  color  as 
that  which  they  absorb. 

*  See  Note  on  §  209. 


APPENDIX.  273 

6.  Refraction  of  Light.  —  The  molecules  of  ether  within  a 
medium  appear  to  be  under  some  constraint,  which  increases 
with  the  density  of  the  medium.     This  causes  the  wave  to  travel 
slower  in  a  dense  than  in   a  rare  medium ;   and  therefore,  on 
entering  a  denser  medium,  an  oblique  ray  of  light  is  bent  towards 
a  perpendicular  to  the  surface  of  the  medium,  while  on  entering 
a  rarer  medium  such  a  ray  is  bent  away  from  the  perpendicular. 
The  quicker  the  vibrations,  the  more  they  are  retarded,  and  the 
more  the  ray  is  refracted.     It  is  owing  to  this  unequal  refrangi- 
bility  that  the  colors  are  spread  out  into  a  spectrum  when  a  ray 
of  light  passes  through  a  prism. 

7.  The  Vibrations  of  Light  are  Transverse. — We  have  seen 
that  a  polarized  ray  of  light  has  sides.     Now,  if  light  were  prop- 
agated, like  sound,  by  longitudinal  vibrations,  it  is  difficult  to 
see  how  a  ray  of  it  could  have  sides>    If  we  take  a  long  rubber 
cord  fastened  at  one  end,  and  alternately  stretch  and  relax  it,  we 
have  a  rude  representation  of  a  ray  of  sound  made  up  of  longi- 
tudinal vibrations.     The  cord  is  evidently  alike  all  round,  and 
has  no  sides.     But  if  we  shake  the  cord  so  that  it  shall  vibrate 
transversely  to  its  length,  we  shall  at  once  see  that  it  is  not  the 
same  above  and  below  as  it  is  to  the  right  and  the  left ;  in  other 
words,  that  it  has  sides.     Hence  we  conclude  that  the  luminous 
vibrations  must  be  transverse.     In  an  unpolarized  ray  the  vibra- 
tions are  transverse,  but  are  executed  in  every  plane ;  so  that 
such  a  ray  is  alike  all  round.     It  is  only  by  forcing  the  vibrations 
all  into  one  plane  that  a  ray  can  be  polarized. 

8.  Heat  and  Light  are  one  and  the  same.  —  We  have  seen  that 
radiant  heat  and  light  are  reflected,  refracted,  and  dispersed  in 
precisely  the  same  way.  It  has  also  been  found  by  difficult  and 
delicate  experiments  that  radiant  heat  is  capable  of  interference 
and  polarization  in  the  same  way  as  light.  These  facts  lead  to 
the  conclusion  that  light  and  heat  are  the  same  thing,  and  the 
following  fact  proves  this  beyond  a  doubt. 

We  have  learned  that  the  solar  spectrum  is  crossed  by  dark 
lines,  known  as  Fraunhofer's  lines.  Now  an  examination  of  the 
spectrum  with  a  very  delicate  thermopile  has  shown  that  these 
dark  lines  are  also  devoid  of  heat,  and,  furthermore,  that  simi- 

18 


274  APPENDIX. 

lar  dark  or  cold  lines  exist  in  the  obscure  part  of  the  spectrum 
beyond  the  red  end,  where  the  heat  is  most  intense.  Again, 
these  dark  lines  have  been  shown  to  be  chemically  inactive,  and 
similar  inactive  lines  are  found  beyond  the  violet  end  in  the 
obscure  chemical  part  of  the  spectrum.  The  existence  of  these 
.blank  lines  throughout  the  whole  length  of  the  spectrum,  in  the 
obscure  as  well  as  in  the  luminous  part,  and  the  absence  of  both 
heat  and  chemical  activity  in  the  dark  lines  found  in  the  lumin- 
ous part,  prove  conclusively  that  the  thermal,  the  luminous,  and 
the  chemical  rays  are  one  and  the  same  thing. 

Passing  from  the  obscure  end  of  the  spectrum  beyond  the  red 
to  the  obscure  end  beyond  the  violet,  we  meet  with  vibrations  of 
greater  and  greater  rapidity,  but  differing  in  nothing  else.  A 
portion  of  these  vibrations  at  the  lower  or  thermal  end  of  the 
spectrum  are  too  slow  to  be  seen,  but  may  be  felt  by  the  nerves 
which  give  us  the  sensation  of  heat.  Another  portion,  including 
the  luminous  part  of  the  spectrum,  can  be  both  seen  and  felt, 
and  can  also  develop  chemical  action.  A  third  portion,  or  those 
beyond  the  violet  end,  are  too  rapid  to  be  seen  or  felt,  but  are 
able  to  cause  chemical  action.  Luminous  heat  and  light,  then, 
are  exactly  the  same  thing;  and  obscure  heat  differs  from  lumi- 
nous heat  only  as  one  color  of  the  spectrum  differs  from  an- 
other. 

If  there  is  need  of  further  proof  that  obscure  heat  differs  from 
light  only  in  the  rapidity  of  the  vibration,  it  is  furnished  by  an 
experiment  of  Dr.  Draper's.  He  gradually  raised  the  tempera- 
ture of  a  platinum  wire  till  it  was  of  a  white  heat,  and  exam- 
ined its  spectrum  throughout  the  process.  At  first  the  spectrum 
contained  only  the  obscure  thermal  rays;  then  the  least  re- 
frangible red  rays  appeared,  followed  in'  succession  by  the 
orange,  yellow,  green,  blue,  indigo,  and  violet;  and  after  these 
came  the  obscure  chemical  rays. 

9.  Electricity  is  a  Mode  of  Molecular  Motion  akin  to  Heat . — 
Suppose  the  liquid  used  with  the  voltaic  pair  to  be  muriatic 
acid.  "The  zinc  plate,  in  virtue  of  the  powerful  affinity  of  zinc 
for  chlorine,  attracts  the  chlorine  atoms,  which  rush  towards  it 
with  immense  velocity;  and  the  sudden  arrest  of  motion  which 


APPENDIX.  275 

attends  the  union  of  the  chlorine  with  the  zinc  has  the  effect  of 
an  incessant  volley  of  atomic  shot  against  the  face  of  the  plate. 
Each  of  the  atomic  blows  must  give  an  impulse  to  the  molecules 
of  the  metal  itself,  which  will  be  transmitted  from  molecule  to 
molecule  through  the  material  of  the  plate  and  the  connecting 
wire,  in  the  same  way  that  a  shock  is  transmitted  along  a  line  of 
ivory  balls."  The  electric  current  is  merely  "  a  wire  or  other  con- 
ductor filled  with  innumerable  lines  of  oscillating  molecules." 

We  know  nothing  of  the  mode  of  the  molecular  motion  in 
the  metallic  conductor.  It  is  apparently  allied  to  heat,  but  is 
capable  of  producing  very  different  effects. 

This  peculiar  mode  of  molecular  motion  may  also  be  de- 
veloped by  heat,  by  magnetism,  and  by  friction  or  percussion. 
In  the  same  way,  heat  may  be  developed,  not  only  by  chemical 
action,  but  also  by  friction  and  percussion. 

10.  The  Molecules  of  all  Bodies  are   in  Motion.  —  It  would 
seem,  then,  that  all  the  molecules  of  gross  matter  are  in  con- 
stant vibration;  and  that,  when  acted  upon  by  heat  or  other 
force,  these  molecules  are  made  to  perform  their  fundamental 
vibrations  with  greater  energy,  and  to  add  to  these  higher  and 
higher  harmonics.     Our  organs  of  sense  are   instruments  for 
intercepting    these   vibrations   and    transmitting    them   to   the 
brain,   where    they  tell   us    nearly  all    that  we    know  of  the 
external  world. 

SOURCES  AND  CONVERSION  OF  ENERGY. 

11.  Kinds  of  Energy. — Every  moving  mass  is  said  to  have 
actual  or  dynamical  energy.     Every  mass  so  situated  that  it  can 
be  moved  by  the  forces  acting  upon  it  is  said  to  have  possible  or 
-potential  energy.     The  energy  of  a  visible  body  in  motion  is 
called  mechanical ;  that  of  moving  molecules,  or  atoms,  is  called 
molecular  or  atomic.     The  energy  manifested   in  the  bodies  of 
animals  is  called  muscular  energy,  or  nerve-force. 

12.  Affinity,  Cohesion,  and  Gravity  tend  to  convert  potential 
into  actual  Energy.  —When  visible  masses  are  separated,  gravity 
tends  to  pull  them  together,  and  to  convert  their  potential  into 


276  APPENDIX. 

dynamical  energy.  When  the  molecules  of  a  body  are  separated, 
as  in  melting  or  boiling,  cohesion  tends  to  draw  them  together 
again,  and  thus  to  convert  their  potential  into  actual  energy, 
which  appears  as  heat.  Again,  when  the  atoms  of  a  body  are 
separated,  affinity  tends  to  bring  them  together  and  to  convert 
their  potential  into  actual  energy,  which  appears,  in  ordinary 
chemical  action,  as  heat  and  electricity,  and,  in  respiration,  as 
heat  and  nerve-force. 

13.  Mechanical  Energy  may  be  converted  into  Heat.ji—VjQ 
have  a  familiar  illustration  of  this  in  the  lighting  of  a  friction 
match.     A  part  of  the  energy  used  in  rubbing  the  match  is  con- 
verted by  the  friction  into  heat,  which  ignites  the  phosphorus. 
Here  there  is  a  double  transfer  of  energy.    The  muscular  energy 
of  the  arm  is  converted  into  mechanical  energy  in  the  moving 
match,  and  a  part  of  this  into  heat  by  the  friction. 

Before  matches  were  invented,  the  flint  and  steel  were  used 
for  the  same  purpose.  The  steel  was  struck  against  the  flint, 
and  the  spark  obtained  was  caught  in  tinder.  A  part  of  the 
mechanical  energy  of  the  steel  appeared  as  heat  in  the  spark. 

Indians  are  s.aid  to  obtain  fire  by  vigorously  rubbing  together 
two  pieces  of  dry  wood.  In  this  case,  too,  the  heat  is  nothing 
but  mechanical  energy  appearing  in  a  new  form. 

Iron  can  be  heated  red-hot  by  hammering  it.  And,  generally, 
heat  is  developed  by  friction  and  percussion. 

14.  All  Mechanical  Energy  is  ultimately  converted  into  Heat. 
—  When  a  falling  body  strikes  the  earth,  it  becomes  heated.     In 
this  case,  the  whole  energy  of  the  body  is  converted  into  heat. 

"When  bodies  are  rubbed  together,  their  energy,  as  we  have  seen, 
is  converted  into  heat. 

The  energy  of  a  running  stream  is  gradually  converted  into 
heat  by  the  friction  against  its  banks  and  bed  and  among  its 
particles.  If  it  is  made  to  turn  the  wheels  of  a  factory  on 
its  way,  the  rubbing  of  the  parts  of  the  machinery  against 
each  other  and  against  the  air,  together  with  the  various  kinds 
of  work  done  by  the  machinery,  converts  the  mechanical  energy 
of  the  water-wheel  into  heat. 

A  railway  train  is  really  stopped  by  the  conversion  of  its 


APPENDIX. 


277 


motion  into  heat.  When  this  has  to  be  done  quickly,  the  change 
is  hastened  by  increasing  the  friction  by  means  of  the  brakes. 
On  the  other  hand,  in  order  to  prevent  the  loss  of  energy  while 
the  train  is  in  motion,  the  axles  of  the  wheels  are  kept  carefully 
oiled,  that  they  may  turn  with  as  little  friction  as  possible. 

When  unlike  substances  are  rubbed  together,  a  part  of  the 
energy  is  first  converted  into  electricity,  but  ultimately  into 
heat. 

15.  When  Mechanical  Energy  is  converted  into  Heat,  the  same 
Amount  of  Energy  always  gives  rise  to  the  same  Amount  of  Heat. 
—  This  was  first  shown  by  Joule,  who  began  his  experiments  in 
1843,  and  continued  them  till  1849.  He  converted  mechanical 
energy  into  heat  by  means  of  friction.  He  first  examined  cases 
of  the  friction  of  solids  against  liquids.  The  apparatus  used  for 
this  purpose  is  shown  in  Figure  211.  B  is  a  cylindrical  box 
holding  the  liquid.  In  the  centre  of  the  box  is  an  upright  axis, 

Fig.  2X1. 


to  which  are  attached  eight  paddles,  like  the  one  shown  in  the 
figure.  These  revolve  between  four  stationary  vanes,  which 
prevent  the  liquid  from  being  carried  round.  The  paddles  are 
turned  by  means  of  the  cord  r  and  the  weight  W.  The  size 
of  the  weight  is  such  that  it  descends  without  acquiring  anjr 


278  APPENDIX. 

velocity,  and  hence  all  its  energy  is  expended  in  the  friction  of 
the  paddles.  The  degree  to  which  the  liquid  becomes  heated 
by  the  friction,  is  shown  by  a  thermometer  at  t.  Knowing  the 
weight  of  the  liquid,  its  specific  heat,  and  the  rise  of  temperature 
during  the  experiment,  the  amount  of  heat  generated  can  be 
readily  calculated. 

With  this  machine  Joule  found  that,  whatever  the  liquid  he 
used,  a  weight  of  one  pound  falling  through  772  feet,  or  772 
pounds  falling  one  foot,  generated  heat  enough  to  raise  one 
pound  of  water  one  degree  Fahrenheit  in  temperature,  or  one 
unit  of  heat,  as  it  is  called. 

He  also  found  that,  when  solids  were  rubbed  together  by  the 
action  of  a  falling  weight,  one  pound  falling  through  772  feet 
generated  a  unit  of  heat.  In  this  experiment,  iron  disks  were 
made  to  rotate  together,  one  against  the  other,  in  a  vessel  of 
mercury. 

If  a  metallic  disk  be  put  into  rapid  rotation,  and  then  brought 
between  the  poles  of  a  powerful  electro-magnet,  it  soon  comes  to 
rest.  It  will  now  be  found  very  difficult  to  turn  it,  and  it  be- 
comes heated  as  it  rotates.  Joule  found  in  this  case,  as  in  the 
others,  that,  if  the  disk  is  turned  by  a  falling  weight,  one  pound 
descending  772  feet  generates  a  unit  of  heat. 

The  force  necessary  to  raise  one  pound  one  foot  is  called  a 
foot-found;  and  this  is  the  same  force  which  a  pound  acquires 
in  falling  one  foot  from  a  state  of  rest. 

We  see,  then,  that  when  mechanical  energy  is  converted  into 
heat,  the  same  amount  of  energy  always  gives  rise  to  the  same 
amount  of  heat,  and  that  772  foot-pounds  of  mechanical  force 
are  equivalent  to  one  unit  of  heat.  For  this  reason,  we  call  772 
foot-pounds  the  mechanical  equivalent  of  heat. 

16.  Heat  may  be  converted  into  Mechanical  Energy.  —  The 
steam-engine  is  a  contrivance  for  converting  heat  into  mechan- 
ical energy.  The  heat  converts  the  water  into  steam,  and  gives 
to  this  steam  an  expansive  force ;  and  this  expansive  force  is 
made  to  move  a  piston,  as  explained  on  pages  91-93. 

The  animal  body  is  a  machine  for  converting  the  molecular 
energy  developed  by  affinity  into  mechanical  energy. 


APPENDIX.  279 

17.  The  Same  Amount  of  Heat  always  gives  rise  to  the  Same 
Amount  of  Mechanical  Energy.  —  In  Figure  212,  C  is  a  box  a 
foot  square.  Suppose  a  a  to  be  a  partition  one  foot  F-  2J2 


from  the  bottom,  so  as  to  shut  in  a  cubic  foot  of 

air.     Suppose  this  partition  to  be  immovable,  and 

the  air  beneath  to  be  heated.     Its  elastic  force  will 

be  increased,  but  it  cannot  expand.     We  will  next 

suppose  that  a  a  is  movable,  but  without  weight, 

and  that  the  air  beneath  is  heated  as  before.     On 

raising  its  temperature  490°,   its  volume  will   be 

doubled,    and   a  a  will  of  course   be   raised   one 

foot  to  b  b.     In  raising  a  a  one  foot,  it  has  had 

to  raise  the  air  above  it.     Now  this  air  presses  with  a  force  of 

15  pounds  upon  every  square  inch,  or  15  x  144  =  2,160  pounds 

upon  the  whole  surface.     From  the  specific  heat  of  air,  we  know 

that  to  raise  the  temperature  of  a  cubic  foot  of  air  490°,  when  it 

is  free  to  expand,  9.5  units  of  heat  are  required. 

But  we  have  seen  that  a  part  of  the  heat  which  enters  a  body 
is  used  in  expanding  it,  and  a  part  in  raising  its  temperature. 
In  the  above  experiment,  how  much  heat  is  used  in  raising  the 
temperature?  This  is  equivalent  to  asking  how  much  heat  is 
required  to  raise  the  cubic  foot  of  air  490°  when  it  is  not  allowed 
to  expand.  It  has  been  found  that  the  computed  velocity  of 
sound  in  air  is  less  than  its  observed  velocity,  and  that  this  is 
owing  to  the  heat  developed  in  the  compressed  portion  of  the 
sound-wave.  From  the  ratio  between  the  observed  and  the 
computed  velocity,  it  is  found  that  the  specific  heat  of  air  when 
free  to  expand  must  be  1.42  of  its  heat  when  not  allowed  to 
expand.  Hence  the  heat  required  to  raise  the  temperature  of 
the  cubic  foot  of  air  490°,  when  it  is  not  allowed  to  expand,  is 
found  by  the  following  proportion  to  be  6.7  units  :  — 
1.42  :  i  =  9.5  :  6.7. 

The  amount  of  heat,  then,  used  in  expanding  the  air — that 
is,  in  raising  2,160  pounds  one  foot  high  —  is  2.8  units.  Divid- 
ing 2,160  by  2.8,  we  get  772,  nearly. 

Since  there  is  no  cohesion  among  the  particles  of  air,  the 
whole  expansive  force  is  used  in  raising  the  weight. 


280  APPENDIX. 

\Ve  see,  then,  that  772  foot-pounds  of  mechanical  force  are 
equivalent  to  a  unit  of  heat,  and  that  a  unit  of  heat  is  equivalent 
to  772  foot-pounds  of  mechanical  force. 

We  have  seen  that  merely  to  melt  a  pound  of  ice  at  a  tem- 
perature of  32°  Fahrenheit  requires  143  units  of  heat,  which  is 
equivalent  to  the  force  required  to  lift  110,396  pounds,  or  about 
55  tons,  a  foot  high.  And  to  convert  a  pound  of  boiling  water 
into  steam  requires  967  units  of  heat,  equivalent  to  the  force 
required  to  lift  746,524  pounds,  or  about  373  tons,  a  foot  high. 
The  force  of  gravity  is  almost  as  nothing  compared  with  this 
molecular  force. 

The  strength  of  affinity  is  shown  by  the  amount  of  heat 
developed  by  the  combination  of  oxygen  and  hydrogen.  It  is 
found  that,  when  oxygen  unites  with  one  pound  of  hydrogen, 
61,000  units  of  heat  are  generated.  Hence  the  force  which  has 
combined  the  two  gases  is  equal  to  61,000  X  772  =  47,092,000  foot- 
pounds, or  the  force  necessary  to  raise  23,546  tons  a  foot  high, 
or  to  throw  one  ton  to  a  height  of  morfi*  than  four  miles.  A 
pound  of  carbon,  in  combining  with  oxygen,  gives  out  about 
14,500  units  of  heat,  equivalent  to  11,194,000  foot-pounds.  We 
see,  then,  that  the  force  even  of  cohesion  is  insignificant  com- 
pared with  that  of  affinity. 

18.  Energy  may  be  transmuted,  but  not  destroyed.  —  We  have 
now  seen  that  mechanical  motion  may  be  converted  into  the 
molecular  motions  of  heat  and  electricity,  and  that  these  molec- 
ular motions  may  be  converted  into  mechanical  motion. 

Energy,  like  matter,  may  assume  a  great  variety  of  forms; 
but,  like  matter,  it  is  wholly  indestructible. 

19.  Source  of  Energy.  —  If  left  to  itself,  affinity  would  soon 
bring  all  dissimilar  atoms  together,  and  lock  them  up  in  com- 
pounds ;  cohesion  would  bring  all  the  molecules  of  these  com- 
pounds together,  and  lock  them  up  in  solids ;  and  gravity  would 
bring  all  these  solids  together,  and  hold  them  in  its  iron  grasp ; 
while  the  heat  developed  by  these  forces  would  be  radiated  into 
space,  and  our  earth  become  one  dreary  waste,  void  of  all  signs 
of  life  and  activity.     What,  then,  is  the  source  of  the  energy 
which  is  thus  manifesting  itself  in  Protean  forms  ? 


APPENDIX.  28l 

Let  us  consider,  first,  the  energy  developed  by  gravity.  This 
energy  is  seen  in  the  winds,  the  falling  rain,  and  running  streams. 
The  atmosphere,  on  each  side  of  the  equator,  is  an  immense 
wheel.  The  side  of  this  wheel  next  the  equator  is  continually 
expanded,  and  thus  made  lighter,  by  the  heat  of  the  sun.  Hence 
gravity  pulls  down  the  colder  and  heavier  side  in  the  polar 
regions,  and  thus  the  wheel  is  made  to  turn.  Were  it  not  for 
the  sun's  heat,  it  would  soon  come  to  rest. 

Again,  the  heat  of  the  sun  evaporates  the  waters  of  the  ocean, 
and  in  their  gaseous  state  they  are  swept  round  with  the  atmos- 
pheric wheel  till  they  come  to  colder  regions,  where  they  are 
condensed,  and  fall  to  the  earth  as  rain,  and  flow  to  the  ocean 
in  rivers.  It  is  due,  then,  to  the  heat  which  comes  to  the  earth 
in  the  sunbeam,  that  gravity  can  thus  unceasingly  manifest  its 
energy. 

The  energy  of  chemical  affinity,  which  is  manifested  in  heat, 
light,  and  muscular  force,  is  developed  by  its  action  between 
oxygen  and  carbon.  How  are  these  elements  separated  from 
carbonic  acid,  so  that  they  may  be  reunited  by  affinity? 

Place  a  leafy  plant  in  a  glass  vessel,  and  let  a  current  of  car- 
bonic acid  stream  over  it  in  the  dark,  and  no  change  takes 
place.  Let  the  same  gas  stream  over  the  plant  in  the  sunshine, 
and  a  part  of  it  will  disappear,  and  be  replaced  by  oxygen. 
When  acted  upon  by  the  sunbeams,  leaves  of  plants  remove 
carbonic  acid  from  the  air,  separate  its  carbon  and  oxygen, 
retain  the  former,  and  give  the  latter  back  to  the  air.  When 
plants  are  consumed  by  combustion  in  our  furnaces,  and  by 
respiration  in  our  bodies,  this  oxygen  combines  with  carbon 
and  develops  energy,  which  appears  as  mechanical  force  in  our 
engines,  and  as  muscular  force  in  our  bodies. 

In  the  summer,  when  more  sunshine  than  we  need  is  poured 
upon  the  earth,  a  part  of  it  is  absorbed  by  the  leaves  of  plants, 
and  used  to  decompose  carbonic  acid,  to  build  up  the  varied 
forms  of  vegetable  life.  In  this  way,  the  forests  and  the  fields 
become  vast  storehouses  of  force  which  has  been  gathered  from 
the  sunbeam.  When,  therefore,  we  burn  fuel  in  our  stoves  and 
food  in  our  bodies,  the  light,  heat,  and  muscular  force  developed 


282  APPENDIX. 

are  only  the  reappearance  in  another  form  of  the  sunbeams 
stored  up  in  plants. 

But  this  process  of  gathering  force  from  the  sunlight  has 
been  going  on  for  ages ;  and  when  we  burn  anthracite  or 
bituminous  coal,  we  are  merely  releasing  the  sunbeams  im- 
prisoned in  plants  which  grew  upon  the  earth  before  it  became 
the  dwelling-place  of  man. 

The  energy  of  affinity,  then,  like  that  of  gravity,  is  nothing 
but  transmuted  sunshine. 

The  only  form  of  energy  known  to  us  which  does  not  come 
to  the  earth  in  the  sunbeam  is  that  developed  by  the  ebb  and 
flow  of  the  tidal  wave.  This  wave  is  dragged  round  the  earth 
mainly  by  the  attraction  of  the  moon ;  and  it  acts  as  a  brake 
upon  the  earth's  rotation,  since  it  is  drawn  from  east  to  west 
while  the  earth  is  turning  from  west  to  east.  The  energy  of 
this  wave,  then,  is  developed  at  the  expense  of  the  earth's 
motion  on  its  axis;  and  it  must  tend  to  retard  this  motion, 
though  to  so  slight  a  degree  that  the  observations  of  thousands 
of  years  have  not  served  to  make  it  appreciable. 

20.  The  Amount  of  Heat  given  out  by  the  Sun.  —  Making  allow- 
ance for  the  heat  absorbed  by  the  atmosphere,  it  has  been  calcu- 
lated that  the  amount  received  by  the  earth  during  a  year  would 
be  sufficient  to  melt  a  layer  of  ice  100  feet  thick  and  covering  the 
whole  earth.    But  the  sun  radiates  heat  into  space  in  every  other 
direction  as  well  as  towards  the  earth ;   and  if  we  conceive  a 
hollow  sphere  to  surround  the  sun  at  the  distance  of  the  earth, 

our  planet  would  cover  only of  its  surface.      Hence 

*    2,300,000,000 

the  sun  radiates  into  space  2,300,000,000  times  as  much  heat 
as  the  earth  receives. 

21.  Source  of  the  Sun's  Heat.  —  It  has  been  supposed  by  some 
that  the  materials  of  the  sun  are  undergoing  combustion,  and 
that  this  combustion  develops  the  light  and  heat  which  it  sends 
forth.     There  are,  however,  no  substances  known  to  us  whose 
burning  would  produce  so  much  heat  for  so  long  a  time  as  we 
know  the   sun  has  been  shining.     Carbon  is  one  of  the  most 
combustible  substances  with  which  we  are  acquainted ;    but  if 
the  sun,  large  as  he  is,  were  a  mass  of  pure  carbon,  and  were 


APPENDIX.  283 

burning  at  a  rate  sufficient  to  produce  the  light  and  heat  that  he 
is  giving  out,  he  would  be  utterly  consumed  in  5,000  years.  It 
seems  hardly  possible,  then,  that  the  solar  light  and  heat  can  be 
generated  by  ordinary  combustion. 

One  of  the  most  satisfactory  theories  of  the  origin  of  the  solar 
heat  is  that  developed  in  1848  by  a  German  physician,  Mayer, 
and  known  as  the  meteoric  theory. 

We  have  seen  that  a  pound-weight  which  has  fallen  through 
772  feet  will,  when  its  motion  is  arrested,  generate  a  unit  of  heat. 
Now,  we  know  that  a  body  falling  that  distance  will  acquire  a 
velocity  of  about  223  feet  a  second.  Hence  a  pound  ball  moving 
with  a  velocity  of  223  feet  a  second  will  generate  a  unit  of  heat 
when  its  motion  is  arrested.  We  know,  too,  that  the  velocity 
with  which  a  falling  body  strikes  the  ground  is  in  proportion  to 
the  square  root  of  the  height  from  which  it  falls;  that  is,  in 
order  to  double  or  treble  its  velocity,  a  body  must  fall  from 
four  or  nine  times  the  height.  A  pound  ball,  then,  moving 
with  a  velocity  of  twice  223  feet  a  second  will  be  able  to  generate 
4  units  of  heat;  one  moving  with  thrice  this  velocity,  9  units  of 
heat ;  and  so  on.  When,  therefore,  we  know  the  weight  of  a 
body  and  the  speed  with  which  it  is  moving,  we  can  easily  cal- 
culate how  much  heat  will  be  generated  on  stopping  it. 

Were  the  earth's  motion  arrested,  its  elements  would  melt 
with  fervent  heat,  and  most  of  them  would  be  converted  into 
vapor.  Were  the  earth  to  fall  into  the  sun,  the  heat  generated 
by  the  shock  would  be  sufficient  to  keep  up  the  solar  light  and 
heat  for  95  years.  We  know  that  countless  swarms  of  meteoric 
bodies  are  revolving  in  rings  about  the  sun,  and  that  they  must 
be  moving  in  a  resisting  medium.  If  so,  they  must  eventually 
be  drawn  into  the  sun,  and,  from  the  velocity  with  which  they 
must  strike,  it  has  been  shown  that  they  could  fall  in  sufficient 
numbers  to  generate  all  the  light  and  heat  of  the  sun,  without 
increasing  his  magnitude  enough  to  be  detected,  since  accurate 
measures  of  his  diameter  were  first  made. 

22.  The  Nebular  Hypothesis. — According  to  Laplace,  the 
material  of  our  solar  system  was  once  a  nebulous  mass  of  ex- 
treme tenuity,  and  the  sun,  moon,  and  planets  were  formed  by 


284  APPENDIX. 

its  gradual  condensation.  Let  us  suppose  such  a  nebulous  mass 
slowly  rotating,  and  gradually  cooling  by  radiation  into  space. 
As  it  cools,  it  must  begin  to  contract;  and  as  it  contracts,  its 
rotation  must  be  quickened,  since  the  matter  at  the  surface  must 
be  moving  faster  than  nearer  the  centre.  It  thus  goes  on  con- 
tracting and  rotating  faster  and  faster,  until  the  centrifugal 
tendency  becomes  so  great  that  cohesion  and  gravity  can  no 
longer  hold  it  together.  A  ring  is  then  detached  from  the 
circumference,  which  continues  to  rotate  by  itself.  The  central 
mass  goes  on  contracting  and  rotating  with  ever-increasing 
velocity,  until  a  second  ring  is  thrown  off.  In  this  way,  ring 
after  ring  is  detached,  and  all  these  rings  continue  to  rotate 
round  the  central  mass  in  the  same  direction.  But  the  rings 
themselves  would  go  on  condensing,  and  at  last  they  would 
be  likely  to  break  up,  each  forming  one  or  several  globular 
masses.  These  would,  of  course,  all  revolve  about  the  central 
mass  in  the  same  direction,  and  their  condensation  would  cause 
them  to  rotate  on  their  axis ;  and  it  has  been  proved  that,  with 
the  exception  of  one  or  two  of  the  outer  ones,  they  must  all 
rotate  on  their  axis  in  the  same  direction  in  which  they  revolve 
in  their  orbits. 

-  But  as  these  masses  condensed,  their  rotation  would  be  ac- 
celerated, and  they  would  be  likely  to  throw  off  rings,  which 
would  either  remain  as  rings,  or  be  condensed  into  globes. 

The  central  mass,  of  course,  forms  the  sun ;  the  rings  which 
it  throws  off,  the  planets;  and  the  rings  thrown  off  by  the 
planets,  the  moons.  In  the  case  of  Saturn,  a  part  of  the  rings 
still  remain  uncondensed,  while  a  part  appear  as  moons. 

The  rings  thrown  off  by  the  central  mass  usually  condensed 
into  one  body,  but,  in  the  case  of  the  minor  planets  and  the 
meteoric  rings,  into  many. 

23.  Helmholtz's  Theory  of  Solar  Heat. —  Helmholtz  has  made 
the  nebular  hypothesis  the  basis  of  his  theory  of  solar  heat,  an 
account  of  which  is  given  by  Tyndall  as  follows  :  — 

"  He  starts  from  the  nebular  hypothesis  of  Laplace,  and, 
assuming  the  nebulous  matter  in  the  first  instance  to  have  been 
of  extreme  tenuity,  he  determines  the  amount  of  heat  generated 


APPENDIX.  285 

by  its  condensation  to  the  present  solar  system.  Supposing  the 
specific  heat  of  the  condensing  mass  to  be  the  same  as  that  of 
water,  then  the  heat  of  condensation  would  be  sufficient  to  raise 
their  temperature  28,000,000°  Centigrade.  By  far  the  greater  part 
of  this  heat  was  wasted  ages  ago  in  space.  .  .  .  Helrnholtz  sup- 
poses this  condensation  to  continue ;  that  a  virtual  falling  down 
of  the  superficial  portions  of,  the  sun  towards  the  centre  still 
takes  place,  a  continual  development  of  heat  being  the  result. 
However  this  may  be,  he  shows  by  calculation  that  the  shrink- 
ing of  the  sun's  diameter  by  .0001  of  its  present  length  would 
generate  an  amount  of  heat  competent  to  cover  the  solar  emis- 
sion for  2,000  years ;  while  the  shrinking  of  the  sun  from  its 
present  mean  density  to  that  of  the  earth  would  have  its  equiv- 
alent in  an  amount  of  heat  competent  to  cover  the  present  solar 
emission  for  17,000,000  of  years." 

Mayer's  theory  is  evidently  not  inconsistent  with  that  of 
Helrnholtz,  but  supplementary  to  it.  The  former  merely  as- 
sumes that  the  meteors  and  planets,  which  were  thrown  off 
from  the  nebulous  mass  as  it  condensed,  are  slowly  falling  into 
it  again.  When  these  shall  all  have  fallen  into  it,  and  the  con- 
densation shall  have  ceased,  our  sun  will  cease  to  shine,  like 
many  other  stars  which  have  disappeared  from  the  heavens. 


FRENCH  WEIGHTS  AND   MEASURES. 

The  names  of  the  higher  orders  of  units,  or  the  multiples 
of  the  standard  unit,  are  formed  from  the  name  of  the  stand- 
ard unit  (the  metre,  litre,  etc.),  by  means  of  prefixes  taken 
from  the  Greek  numerals;  namely,  deca-  (10),  hecto-  (100),  kilo- 
(1,000). 

The  names  of  the  lower  orders  of  units,  or  the  subdivisions  of 
the  standard  unit,  are  formed  in  a  similar  manner  by  means 
of  prefixes  taken  from  the  Latin  numerals ;  namely,  deci-  (10), 
centi-  (100),  milli-  (1,000). 


286  APPENDIX. 


TABLE  OF  LINEAR  MEASURE. 


io  millimetres 

=  i  centimetre 

=     0.3937  inch. 

10  centimetres 

=  i  decimetre 

=     3-937      " 

io  decimetres 

=  i  metre 

=    39-37 

io  metres 

=  i  decametre 

=  393-7 

io  decametres 

=  i  hectometre 

=  328   ft.    i  inch. 

io  hectometres 

=  i  kilometre 

=  3280  "  io    " 

TABLE  OF  MEASURES  OF  SURFACE, 
loocentiares  =  i  are  =  119.6  square  yards. 

loo  ares  =  i  hectare       =      2.471  acres. 

The  centiare  is  a  square  metre,  or  1,550  square  inches. 

TABLE  OF  MEASURES  OF  CAPACITY. 

10  millilitres  =  i  centilitre    =      0.6102  cubic  inches. 

10  centilitres  =  i  decilitre      =      6.1022      "         " 

10  decilitres  =  i  litre  —      1.0567  wine  quarts. 

10  litres  =  i  decalitre     =      2.6417     "     gallons. 

10  decalitres  =  i  hectolitre    =    26.417       "          " 

10  hectolitres  =  i  kilolitre      =264.17         "          " 
The  kilolitre  is  a  cubic  metre,  and  is  also  called  a  stere.    The 
decastere  ==•  10  steres. 

TABLE  OF  WEIGHTS. 

10  milligrammes     =  i  centigramme     =    0.1543  grains. 
10  centigrammes    =  i  decigramme       =    1.5432      " 
10  decigrammes      =  i  gramme  =  15.432        " 

10  grammes  =  i  decagramme      =    0.3527  oz.  avoirdupois. 

10  decagrammes     =  i  hectogramme    =    3-5274  "  " 

10  hectogrammes   =  i  kilogramme       =    2.2046  pounds  " 

The  millier  or  tonneau  is  equal  to  1,000,000  grammes,  or  2204.6 
pounds  avoirdupois. 


The  English  equivalents  given  above  are  those  which 
were  established  by  Congress,  in  July,  1866,  and  are  sufficiently 
accurate  for  all  practical  purposes. 


APPENDIX.  287 


PROBLEMS. 

Teachers  can  use  either  the  English  or  the  French  weights  and  measures, 
or  both.  The  French  weights  are  not  equivalents  of  the  English,  unless  the  nature  of 
the  problem  requires  it. 

WEIGHT  OF  I^IQUIDS.  —  i.  A  glass  flask,  when  full  of  water, 
weighs  9  ounces  (180  grammes).  The  flask  itself  weighs  4.2 
ounces  (84  grammes).  How  many  ounces  (grammes)  of  water 
does  the  flask  hold? 

2.  The  same  flask,  when  full  of  mercury,  weighs  69.1  ounces, 
(1,382  grammes).     How  many  ounces  (grammes)  of  mercury 
does  it  hold? 

3.  The    same   flask,   full   of    alcohol,  weighs  8  ounces    (160 
grammes).      How  many  ounces  (grammes)  of  alcohol  does  it 
hold? 

4.  The  same  flask,  full  of  sulphuric   acid,  weighs  n  ounces 
(220  grammes).     How  many  ounces  (grammes)   of  sulphuric 
acid  does  it  hold? 

THE  PRESSURE  WHICH  LIQUIDS  EXERT  BY  REASON  OF  THEIR 
WEIGHT.  —  In  these  problems,  it  is  assumed  that  in  liquids  the 
pressure  increases  at  exactly  the  same  rate  as  the  depth. 

5.  When  water  is  one  foot   (centimetre)  deep  in  a  vessel,  it 
exerts  a  pressure  of  62.5  Ibs.  (one  gramme)  on  every  square 
foot  (centimetre)  of  surface  at  the  bottom  of  the  vessel.     What 
would  be  the  pressure  exerted  upon  every  square  foot  (centi- 
metre) of  surface  at  the  bottom,  if  the  water  in  the  vessel  were 
3  feet  (centimetres)  deep  ? 

6.  What  would  be  the  pressure  upon  9  square  feet  (decimetres) 
of  surface  at  the  bottom,  if  the  liquid  were  6  feet  (centimetres) 
deep  ? 

7.  What   upon  13  square  feet  (decimetres)  at  the  bottom,  if 
the  liquid  were  7.5  feet  (17  centimetres)  deep? 

8.  A  closed  vessel  is  9  inches  (3  decimetres)  deep,  and  has  a 
tube  projecting  from  the  top  to  the  height  of  one  yard  (metre). 
The  bottom  of  the  vessel  has  a  surface  of  100  square  inches  (50 
square  decimetres),  and  the  vessel  is  filled  with  water  to  the  top 


288  APPENDIX. 

of  the  tube.    What  is  the  whole  pressure  upon  the  bottom  of  the 
vessel ? 

9.  What  would  be  the  pressure  upon  a  square  inch  (centimetre) 
of  surface  on  the  side  of  the  above  vessel,  the  centre  of  the  sur- 
face being  3  inches  (centimetres)  from  the  bottom  ? 

10.  What  would  be  the  pressure  upon  a  square  inch  (centi- 
metre) of  surface  at  the  top  of  the  vessel? 

11.  What  would  be  the  pressure  upon  the  whole  upper  surface 
of  the  vessel,  supposing  it  to  contain  100  square  inches  (50  square 
decimetres)  ? 

12.  A  cubical  vessel,  every  side  of  which  is   a   square  yard 
(metre),    is   filled  with  water.      What  would   be   the  pressure 
upon  its  bottom? 

13.  What  would  be  the  pressure  upon  each  of  its  sides  ?  * 

14.  Suppose  the  top  of  the  above  vessel  were  closed,  and  a 
tube  one  yard  (metre)  in  length  were  inserted  into  it,  and  were 
filled  with  water,  what  would  be  the  pressure  exerted  upon  the 
top  of  the  vessel  ? 

15.  What  would  be  the  pressure  upon  the  bottom  of  the  vessel 
when  the  tube  is  full  of  water? 

16.  What  would  be  the  pressure  upon  the  sides  of  the  vessel  in 
the  last  case  ? 

THE  HYDROSTATIC  PRESS.  — 17.  The  end  of  the  small  piston 
in  a  hydrostatic  press  has  a  surface  of  10  square  inches  (centi- 
metres) ;  and  the  end  of  the  large  piston  a  surface  of  a  square 
foot  (decimetre).  A  pressure  of  10  pounds  (kilogrammes)  upon 
the  small  piston  would  bring  what  pressure  to  bear  upon  the 
large  piston? 

18.  If  the  small  piston  be  the  same  as  above,  and  the  end  of 
the  large  piston  contain  a  square  yard  (metre)  of  surface,  5 
pounds  (kilogrammes)  upon  the  small  piston  will  cause  what 
pressure  to  be  brought  to  bear  upon    the  end   of  the   large 
piston  ? 

19.  A  pressure  of  75  pounds  (kilogrammes)    on  the   small 

*  To  find  the  pressure  upon  any  surface  at  the  sides  of  a  vessel,  take  the  average 
depth  of  the  surface,  that  is,  the  distance  from  the  top  of  the  water  to  the  middle 
of  that  surface. 


APPENDIX.  289 

piston  would  cause  what  pressure  to  be  exerted  upon  the  end 
of  the  large  piston  ? 

THE  BUOYANCY  OF  LIQUIDS. — A  cubic  inch  of  water  weighs 
252.458  grains,  the  grain  being  y^-^  of  a  pound  avoirdupois. 
For  ordinary  calculations,  a  cubic  foot  of  water  may  be  assumed 
to  weigh  1,000  ounces,  or  62.5  pounds,  avoirdupois.  A  cubic 
centimetre  of  water  weighs  I  gramme. 

20.  A  body  weighs  50  pounds  (kilogrammes)  in  air,  and  has  a 
bulk  of  432  cubic  inches  (40  cubic  decimetres).    How  much  does 
it  weigh  in  water  ? 

21.  A  stone  weighs  80  pounds  (kilogrammes)  in  the  air,  and 
55  pounds  (kilogrammes)  in  water.     What  is  its  bulk? 

22.  A  hollow  vessel  of  copper  weighs  one  pound  (kilogramme). 
What  must  be  its  bulk  in  order  that  it  may  just  float  in  water? 

23.  A  hollow  vessel  of  iron  weighs  15  pounds  (kilogrammes). 
What  must  be  its  bulk  in  order  that  it  may  sink  one-half  in 
water  ? 

24.  A  boat  displaces  12  cubic  yards  (metres)  of  water.    What 
is  its  weight? 

SPECIFIC  GRAVITY.  —  25.  A  body  weighs  15  pounds  (150  hec- 
togrammes) in  air,  and  weighs  2  pounds  (2  kilogrammes)  in 
water.  What  is  the  weight  of  a  bulk  of  water  equal  to  that  of 
the  body? 

26.  A  flask  full  of  water  weighs  6.2  ounces  (62  grammes) :  a 
piece  of  lead  weighs  44  ounces  (44  decagrammes)  in  the  air.    It 
is  put  into  the  flask,  and  the  flask  is  filled  with  water.      It  is 
found  that  the  lead  and  water  together  weigh  46.2  ounces  (462 
grammes).     What  is  the  weight  of  a  bulk  of  water  equal  to  that 
of  the  lead  ? 

27.  A  piece  of  lead  weighs  3  pounds,  8  ounces  (56  grammes)  in 
the  air,  and  3  pounds,  3  ounces  (51  grammes)  in  water.    What  is 
the  specific  gravity  of  lead  ? 

28.  A  flask  holds  15  ounces  (75  grammes)  of  water :  a  lump  of 
copper,  which  weighs  i  pound  (160  grammes)  in  the  air,  is  put 
into  the  flask,  and  it  is  found  that  the  water  and  the  copper 
together  weigh  i  pound,  5.9  ounces  (219  grammes).     What  is  the 
specific  gravity  of  copper? 

'9 


290  APPENDIX. 

29.  The  specific  gravity  of  iron  is  7.8.    What  weight  of  water 
will  45  pounds  (kilogrammes)  of  iron  displace? 

30.  The  specific  gravity  of  zinc  is  7.2.     What  is  the  bulk  of  90 
pounds  (kilogrammes)  of  zinc? 

31.  A  piece  of  wood  which  weighs  5  ounces  (25  grammes)  in 
the  air,  is  fastened  to  a  piece  of  iron  whose  weight  is  i  pound 
(80  grammes) ;  and  on  immersing  both  in  water  and  weighing 
them,  it  is  found  that  they  together  weigh  9  ounces  (45  grammes). 
What  is  the  weight  of  the  water  displaced  by  the  wood? 

32.  A  piece  of  wood,  weighing  4.2  ounces  (42  grammes),  is 
fastened  to  a  piece  of  zinc  weighing  8.6  ounces  (86  grammes), 
and  both  are  weighed  under  water,   and   are  found  to  weigh 
3.4  ounces  (34  grammes).     What  is  the  specific  gravity  of  the 
wood? 

33.  A  flask  weighing  2  ounces  (20  grammes)  weighs  2  pounds, 
ii  ounces  (430  grammes)  when  full  of  water,  and  34  pounds,  11.5 
ounces  (5555  grammes)  when   full  of  mercury.     What  is  the 
specific  gravity  of  mercury  ? 

34.  A  hydrometer  weighing  5  ounces  (50  grammes)  requires  a 
weight  of  8  ounces  (80  grammes)  to  sink  it  to  the  neck  in  water, 
and  a  weight  of  13.5  ounces  (135  grammes)  to  sink  it  to  the  same 
depth  in  sulphuric  acid.  What  is  the  specific  gravity  of  sulphuric 
acid? 

35.  A  vessel  holds  100  pounds  (kilogrammes)  of  water.     How 
much  mercury  would  it  hold  ? 

36.  How  much  alcohol  will  it  hold,  if  the  specific  gravity  of 
alcohol  is  .79? 

WEIGHT  OF  GASES.— The  specific  gravity  of  a  gas  is  its 
weight  compared  with  that  of  an  equal  bulk  of  atmospheric 
air. 

37.  A  glass  globe  of  the  capacity  of  61  cubic  inches  (one  litre) 
weighs  29.27  ounces  (83  grammes)  after  the  air  has  been  ex- 
hausted from  it;  and  29.73  ounces  (84.292  grammes)  when  full 
of  air.     What  is  the  weight  of  61  cubic  inches  (a  litre)  of  air? 

38.  The  same  globe,  when  full  of  ammonia  gas,  weighs  29.54 
ounces  (83.759  grammes).   What  is  the  weight  of  61  cubic  inches 
(a  litre)  of  ammonia  gas? 


APPENDIX.  291 

39.  The  same  flask,  when  full  of  carbonic  acid,  weighs  29.96 
ounces  (84.964  grammes).   What  is  the  weight  of  61  cubic  inches 
(a  litre)  of  carbonic  acid  ? 

40.  The   same  flask,   full   of  hydrogen,  weighs   29.3  ounces 
(83.089  grammes).     What  is  the  weight  of  61  cubic  inches  (a 
litre)  of  hydrogen? 

41.  What  is  the  specific  gravity  of  ammonia  gas? 

42.  What  is  the  specific  gravity  of  carbonic  acid  ? 

43.  What  is  the  specific  gravity  of  hydrogen? 

44.  A  vessel  of  the  capacity  of  34  cubic  feet  (985  litres)  would 
hold  how  many  ounces  (grammes)  of  air?  of  carbonic  acid?  of 
hydrogen  ? 

PRESSURE  CAUSED  BY  THE  WEIGHT  OF  GASES.  —  The  atmos- 
pheric pressure  (74)  is  about  one  kilogramme  upon  every  square 
centimetre  of  surface  at  the  level  of  the  sea. 

45.  The  body  of  an  ordinary-sized  man  has  a  surface  of  about  * 
2,340  square  inches  (16,000  square  centimetres).      How  many 
pounds  (kilogrammes)  of  pressure  does  the  atmosphere  exert 
upon  a  man's  body? 

46.  A  room  is  12  yards  (metres)  long,  9  yards  (metres)  wide, 
and  5  yards  (metres)  high.      How  many  pounds  (kilogrammes) 
of  pressure  does  the  atmosphere  exert  upon  the  floor  of  the  room  ? 

47.  How  many  pounds  (kilogrammes)  of  pressure  does  it  exert 
upon  each  end  of  the  room  ?     How  many  on  each  side  ? 

48.  How  many  pounds  (kilogrammes)  of  air  does  the  room 
contain  ? 

49.  The  atmospheric  pressure  will  balance  a  column  of  mer- 
cury 30  inches  (76  centimetres)  high,  and  the  specific  gravity  of 
mercury  is  13.5.     It  will  balance  a  column  of  water  how  many 
feet  (metres)  high  ? 

50.  If  water  is  to  be  raised  45  feet  (1,200  centimetres)  high  by 
means  of  the  lifting-pump,  how  much  of  this  distance  must  the 
water  be  lifted  ? 

51.  Water  is  to  be  carried  over  a  hill  68  feet  (1,350  centimetres) 
high?     Can  it  be  done  by  means  of  the  siphon?    Why? 

BUOYANCY  OF  GASES.  —  52.  A  block  of  wood  has  a  bulk  of 
900  cubic  yards  (metres).     How  much  is  it  buoyed  up  in  the  air? 


292  APPENDIX. 

53.  A  balloon  when  filled  with  gas  weighs  1,000  pounds  (500 
kilogrammes).     How  many  cubic  feet  (litres)  of  bulk  must  it 
have,  in  order  that  it  may  just  float  in  the  air? 

54.  A  balloon  has  a  bulk  of  1,000  cubic  yards  (metres),  and 
weighs  50  pounds  (25  kilogrammes).     It  is  filled  with  coal-gas, 
whose  specific  gravity  is  .6.    By  how  many  kilogrammes  of  pres- 
sure is  it  forced  upward?    If  a  car,  which,  with  all  its  fixtures, 
weighs  96  pounds  (48  kilogrammes),  be  attached  to  the  balloon, 
with  what  pressure  will  the  whole  be  forced  upward  ? 

SECOND  LAW  OF  MOTION.  —  Gravity  causes  a  body  to  fall 
from  a  state  of  rest  4.9  metres  in  a  second,  and  increases  its 
velocity  9.8  metres  in  a  second  (95). 

55.  A  body  falls  from  a  state  of  rest.    What  will  be  its  velocity 
at  the  end  of  the  third  second  ? 

56.  A  body  is  thrown  downward  with  a  velocity  of  50  yards 
(metres)  a  second.    What  will  be  its  velocity  at  the  end  of  7 
seconds  ? 

57.  A  body  is  thrown  downward  with  a  velocity  of  23  yards 
(metres)  a  second.     What  will  be  its  velocity  at  the  end  of  9 
seconds  ? 

58.  A  body  is   thrown   upward  with   a  velocity  of  42  yards 
(metres)  a  second.     What  will  be  its  velocity  at.  the  end  of  4 
seconds  ?    At  the  end  of  6  seconds  ? 

59.  A  body  is  thrown  upward  with   a  velocity  of  98  yards 
(metres)  a  second?    How  long  will  it  continue  to  rise? 

60.  How  high  will  the  above  body  rise  ? 

61.  How  far  will  it  rise  the  first  3  seconds? 

62.  How  far  will  it  rise  the  last  3  seconds  ? 

63.  How  far  will  it  rise  from  the  beginning  of  the  3d  to  the 
end  of  the  8th  second. 

64.  Two  bodies  are  thrown  upward,  one  with  a  velocity  of  224 
feet  (68.6  metres)  a  second,  and  the  other  with  a  velocity  of  448 
feet  (137.2  metres)   a  second.     How  many  seconds  will   it  be 
before  each  begins  to  fall? 

65.  To  what  height  would  each  rise  ? 

66.  A  ball  falls  from  a  state  of  rest,  and  reaches  the  earth  in 
12  seconds.    With  what  velocity  does  it  strike  the  earth? 


APPENDIX.  293 

67.  From  what  height  did  the  ball  in  the  last  example  fall  ? 

68.  How  far  did  it  fall  the  first  5  seconds? 

69.  How  far  did  it  fall  the  last  5  seconds? 

70.  How  far  did  it  fall  from  the  beginning  of  the  3d  to  the 
end  of  the  5th  second  ? 

71.  How  far  did  it  fall  from  the  beginning  of  the  8th  to  the 
end  of  the  nth  second? 

72.  A  ball  is  thrown  downward  with  a  velocity  of  125  yards 
(metres)  a  second,  and  reaches  the  earth  at  the  end  of  7  seconds. 
What  is  its  velocity  on  reaching  the  earth  ? 

73.  From  what  height  was  the  ball  in  the  last  example  thrown  ? 

74.  Through  what  distance  did  it  pass  from  the  beginning  of 
the  3d  to  the  end  of  the  6th  second  ? 

75.  A  stone  falls  from  a  state  of  rest,  and  is  4  seconds  in 
reaching  the  earth.     With  what  velocity  does  it  strike  the  earth? 
Through  what  distance  does  it  fall  ? 

76.  If  the  stone  had   reached  the  earth  in  8  seconds,  what 
velocity  would   it   have   acquired,    and  through  what   distance 
would  it  have  fallen? 

77.  If  the  stone  had  reached  the  earth  at  the  end  of  12  seconds, 
with  what  velocity  would  it  have  reached  the  earth,  and  through 
what  distance  would  it  have  fallen  ?  * 

78.  A  body  in  falling  from  a  state  of  rest  through   16  feet 
(4.9  metres)  acquires  a  velocity  of  32  feet  (9.8  metres)  a  second. 
Through  what   distance   must  it  fall   in   order  to   double  this 
velocity?    To  treble  this  velocity? 

79.  A  stone  falls  from  a  height  of  64  feet  (19.6  metres).     With 
what  velocity  does  it  reach  the  earth  ? 

THIRD  LAW  OF  MOTION.  — To  find  the  momentum  of  a  body, 
multiply  its  weight  in  founds  {grammes)  by  its  velocity  in  feet 
(metres). 

*  When  we  know  the  velocity  a  body  acquires  in  falling  through  a  certain  distance 
a,  and  we  wish  to  know  what  velocity  it  will  acquire  in  felling  through  any  other  dis- 
tance 6,  divide  the  distance  b  by  a,  extract  the  square  root  of  the  quotient,  and  multiply 
the  velocity  the  body  acquires  in  felling  through  the  distance  a  by  the  number  thus 
obtained.  If,  on  the  other  hand,  we  wish  to  know  how  far  the  body  must  fall  to  acquire 
any  velocity  c,  divide  the  velocity  c  by  the  velocity  a  body  acquires  in  felling  through 
the  distance  a,  square  the  quotient,  and  multiply  the  distance  a  by  this  number. 


294  APPENDIX. 

80.  A  body  weighs  50  pounds  (kilogrammes),  and  is  moving 
at  the  rate  of  12  yards  (metres)   a  second.    What  is  its  mo- 
mentum? 

81.  The  same  body  is  moving  at  the  rate  of  5  yards  (metres)  a 
second.   What  is  its  momentum? 

82.  With  what  velocity  must    a    body  weighing    6    pounds 
(grammes)  move,  in  order  to  have  the  same  momentum  as  a 
body  weighing  10,000  pounds  (500  kilogrammes),  and  moving 
at  the  rate  of  2  yards  (metres)  a  second  ? 

83.  A  certain  force  gives  to  a  body  weighing  45  pounds  (kilo- 
grammes)   a   velocity  of   9  yards    (metres)    a   second.      What 
velocity  would   the   same  force    give    to    a  body  weighing   3 
ounces    (grammes)  ? 

MACHINES.  —  84.  In  a  lever,  the  short  arm  is  5  inches  (deci- 
metres) long,  and  the  long  arm  61  inches  (decimetres)  long. 
How  far  will  the  end  of  the  long  arm  move  while  the  end  of 
the  short  arm  moves  through  3  inches  (centimetres)  ? 

85.  In  a  lever,  the  short  arm  is  2  yards  (metres)  long,  and  the 
long  one  15  feet  (50  decimetres)  long.      A  power  of  2  pounds 
(kilogrammes)   is  applied  to  the  end  of  the  long  arm.     What 
weight  at  the  end  of  the  short  arm  will  it  balance? 

86.  While  the  weight  in  the  last  example  is  moving  through  3 
inches  (decimetres),  how  far  will  the  power  move? 

87.  A  weight  of  5  pounds  (60  decagrammes)  is  applied  at  the 
end  of  the  long  arm  of  the  lever  in  the  above  example.     What 
power  must  be  applied  at  the  end  of  the  short  arm  to  balance  it? 

88.  In   a  wheel  and  axle,  the  circumference  of  the  wheel  is 
4  yards  (metres)  and  that  of  the  axle  9  inches  (30  centimetres). 
What  weight  will  a  power  of  3  pounds  (grammes)  balance? 

89.  In  a  train  of  wheels,  a  power  of  i  ounce  (gramme)  balances 
a  weight  of  450  pounds  (43  kilogrammes).     What  distance  must 
the  power  move  through  while  the  weight  moves  through  12  feet 
(50  decimetres)  ? 

90.  In  a  system  of  pulleys,  a   power  of  I   ounce  (gramme) 
balances  a  weight  of  1,200  pounds  (245  kilogrammes).      How 
far  will  the  weight  move  while  the  power  is  moving  through 
i  foot  (metre)? 


APPENDIX.  295 

NOTES   ON  EXPERIMENTS. 

In  the  following  notes,  we  shall  mention  all  the  apparatus 
necessary  for  performing  nearly  all  the  experiments  in  this 
book.  In  the  larger  Natural  Philosophy  of  the  Cambridge 
Physics,  many  additional  pieces  of  apparatus  are  described,  and 
many  additional  experiments  and  illustrations  are  given.  The 
teacher  should  have  that  book  for  reference  and  for  use  in  oral 
instruction. 

For  priced  list  of  apparatus,  see  the  end  of  the  book.  This 
list  has  been  very  carefully  prepared  by  Mr.  Ritchie,  under  the 
supervision  of  the  authors.  The  articles  whose  prices  are  car- 
ried out  in  the  right-hand  column  are  all  that  are  really  neces- 
sary for  the  thorough  illustration  of  this  book.  The  others  are, 
however,  very  desirable,  if  one  has  the  means  to  get  them. 

COHESION.  — i.  Ring  and  ball,  for  §  3.  A  bar  and  gauge 
will  perhaps  be  better  for  showing  contraction  and  expansion 
by  change  of  temperature. 

2.  Two  half-pint  flasks,  provided  with  rubber  corks,  through 
which  glass  tubes  of  small  bore  pass.     These  can  be  used  in  all 
cases,  instead  of  a  bulb  with  projecting  tube ;  and  the  flasks  will 
be  useful  for  many  other  experiments. 

3.  A  pair  of  lead  hemispheres,  for  §  9. 

4.  Two  evaporating  dishes ;  for  making  crystals,  and  for  other 
purposes. 

5.  A  crucible,  for  sulphur  crystals. 

6.  A  dozen  Rupert's  drops. 

7.  Two  glass  two-quart  jars,  with  ground  mouths  and  plates 
to  cover  them. 

8.  A  dropping-tube.     This  is  to  be  used  in  the  experiment  in 
§  21,  which  we  advise  all  teachers  to  try.     Mix  half  the  desired 
quantity  of  alcohol  and  water  to  a  density  near  that  of  the  oil, 
but  a  little  less,  so  that  a  drop  of  oil  will  slowly  sink  in  it;  and 
mix  the  other  half  to  a  density  a  little  greater  than  that  of  the 
oil,  so  that  a  drop  of  oil  will  slowly  rise  in  it.    Then  pour  the 


296  APPENDIX. 

heavier  mixture  under  the  lighter,  through  a  long  tube.  Next 
fill  the  bulb  and  stem  of  the  dropping-tube  with  sweet  oil,  close 
the  top  with  the  thumb,  and  put  the  small  end  into  the  centre 
of  the  mixture.  Remove  the  thumb,  and  keep  the  tube  steady 
till  the  oil  runs  out. 

If  the  mixture  is  made  in  one  of  the  two-quart  jars  (No.  7) 
and  covered,  the  sphere  of  oil  can  be  kept  for  a  number  of 
days. 

9.  Half  a  dozen  6-inch  test-tubes,  one  of  which  is  to  have  a 
rubber  cork  and  tube  for  the  experiment  in  §  22. 

10.  A  U-tube  and  nipper-tap,  for  §§  23  and  24. 

11.  Wooden  retort-holder,  for  holding  U-tube  and  for  other 
purposes. 

12.  Eight  or    ten   pounds   of  mercury  for  experiments  with 
U-tube,  and  also  for  experiments  in  §§  27,  36,  40,  and  72. 

ADHESION. —  i.  Glass  disk  with  hook,  for  the  experiment  in 

§27- 

2.  A  shallow  glass  dish. 

3.  Two  small  glass  funnels,  and  a  pack  of  filters. 

4.  A  set  of  capillary  tubes. 

5.  A  pair  of  capillary  plates. 

6.  Two  glass  cylinders :  one,  i  inch  in  diameter  and  5  inches 
deep;  the  other,  i£  in  diameter  and  7  deep.    These  should  have 
ground  mouths  and  ground-glass  plates  for  covers.     The  small 
one  is  to  be  used  for  §§  36  and  40,  and  the  large  one  for  §  38. 

For  method  of  filling  the  cylinder  with  ammonia,  see  Hand- 
look  of  Chemistry,  page  189,  §  23.  Do  not  fail  to  try  these  experi- 
ments. 

7.  Glass  tube,  about  10  inches  long ;  to  be  used,  with  a  small 
funnel,  for  experiment  in  §  38,  which  is  simple  and  striking. 

8.  Bladder  and  tube,  for  §  39.     The  tube  should  have  as  small 
a  bore  as  possible,  that  the  rise  of  the  liquid  may  be  quickly 
seen.   With  proper  care,  the  alcohol  can  be  poured  in  through  a 
small  funnel. 

A  better  form  of  apparatus  would  be  a  glass  bell,  with  a  tube 
projecting  from  its  top;  the  bladder  being  stretched  over  the 
mouth  of  the  bell. 


APPENDIX. 


297 


9.  Bottles  and  tube,  for  §  42. 

For  the  preparation  of  hydrogen,  see  Handbook  of  Chemistry -, 
page  182,  §  ii.  For  preparation  of  carbonic  acid,  see  same  book, 
page  29,  §  42.  For  the  method  of  filling  the  bottles  with  the 
gases,  see  same  book,  page  181,  §  i. 

The  upper  bottle  and  tube  should  be  together  filled  with 
hydrogen.  The  end  of  the  tube  should  then  be  closed  with  the 
thumb  and  inserted  into  the  other  bottle.  The  cork  of  the  lower 
bottle  should  be  left  on  the  tube. 

10.  Cup  and  tube,  for  §  43.     Instead  of  a  bell-jar,  one  of  the 
jars  in  No.  7  of  Cohesion  may  be  used. 

The  last  two  experiments  are  very  striking,  and  can  be  easily 
performed. 

MECHANICS.  —  i.  Set  of  balls,  rods,  etc.,  for  illustrating  centre 
of  gravity. 

2.  Bottle  with  tubes  (Figure  26). 

3.  Equilibrium  tubes  (Figure  27). 

4.  Illustration  of  hydrostatic  press. 

5.  Hydrometer  (Figure  34). 

6.  Cylinder  and  cup,  for  §  62. 

7.  An  air-pump.     For  all  the  experiments  in  this  book,  a 
table  air-pump  will  answer. 

8.  Receiver,  with  sliding-rod. 

9.  Quart  receiver  and  small  bolt-head. 

10.  Apparatus  for  illustrating  weight  and  buoyancy  of  air. 
n.  Magdeburg  hemispheres. 

12.  Hand-glass. 

13.  Small  rubber  bag. 

14.  Barometer  tube. 

15.  Model  of  lifting  pump. 

16.  Model  of  force  pump. 

17.  Glass  siphon. 

18.  Tantalus's  cup. 

19.  Condenser. 

20.  Condensing  chamber,  with  air-gun  and  jets. 

21.  Guinea  and  feather  tube. 

22.  Illustration  of  pendulum. 


298 


APPENDIX. 


Fig.  214.  23.  Illustration  of  first    law  of 

motion  (Figure  214).  When  the 
ring  is  in  rapid  rotation,  it  as- 
sumes the  form  shown  bj  the  dotted 
line,  in  consequence  of  the  ten- 
dency of  its  parts  to  move  in 
straight  lines,  or  what  is  usually 
called  the  centrifugal  force. 

24.  Models    of   lever   and    com- 
pound lever. 

25.  Models  of   wheel    and   axle, 
capstan,  and   pulleys;   with  stand 
and  weights. 

26.  Models  of  inclined  plane,  wedge,  and  screw. 

27.  Model  of  Barker's  mill. 

28.  Wollaston's  illustration  of  low-pressure  engine. 

SOUND.  —  i.  Bell,  for  §  144.  A  sliding- rod  is  not  necessary, 
as  the  bell  can  be  rung  by  tilting  the  pump. 

2.  Toothed  wheel,  for  §  155.     If  one  has  an  ordinary  gyro- 
scope, a  toothed  wheel  can  be  readily  fitted  to  its  axle. 

3.  Tuning-fork  and  sounding-box. 

4.  Sonometer  and  strings.     This  is  the  most  important  piece 
of  apparatus  for  sound. 

"In  Ritchie's  Improved  Sonometer  the  case  is  of  mahogany, 
thirty-six  inches  in  length,  with  sounding-board  of  spruce,  fitted 
for  two  wires,  with  tension-keys  and  wrench,  and  a  brass  lever 
with  two  weights,  for  measuring  tension.  The  upper  line  of  fig- 
ures on  the  lever  is  for  the  smaller,  and  the  lower  line  for  the 
greater  weight.  There  are  two  scales  marked  on  the  instru- 
ment :  one,  the  diatonic  scale,  with  letters  and  syllables  for  the 
intervals  of  tones  and  semitones,  and  the  ratio  of  length  of 
chord,  and  number  of  vibrations ;  and  the  other,  a  scale  of 
sixty  equal  parts.  There  are  also  movable  bridges  for  one  or 
both  wires  to  rest  upon. 

"To  produce  the  notes  of  the  scale,  move  the  bridge  to  the 
letters  on  the  scale,  and  sound  with  the  bow. 

"  To  show  that  the  rapidity  of  vibration  is  as  the  square  root 


APPENDIX.  2QQ 

of  the  tension,  attach  one  wire  to  the  lever,  place  the  weight 
on  some  number,  tighten  the  wire  until  the  lever  is  brought  to 
a  level,  and  tune  the  other  wire  to  unison.  Then  change  the 
weight  to  some  number  on  the  lever  corresponding  to  a  chord. 
A  change  from  2  to  8  will  give  an  octave ;  i  to  16,  two  octaves ; 
4  to  9,  a  fifth  ;  etc. 

"  To  show  that  the  rapidity  of  vibration  is  inversely  as  the  square 
root  of  the  'weighty  place  the  large  wire,  which  is  four  times  as 
heavy  as  the  other,  on  the  lever,  and  the  weight  at  16;  tune  the 
other  wire  to  unison,  then  change  the  large  wire  for  one  of  the 
same  size  as  the  second  one,  and  raise  the  weight  as  before. 
The  new  wire  will  sound  an  octave  higher  than  the  other. 
Place  the  weight  at  4,  and  it  will  give  the  unison. 

"  If  the  weight  be  changed  considerably,  it  will  be  necessary  to 
tighten  or  loosen  the  screw  to  allow  for  the  stretching  of  the 
wire. 

"  For  producing  overtones,  or  harmonics,  touch  the  wire  with  a 
feather,  or  lightly  with  the  finger,  at  one  of  the  equal  divisions, 
and  draw  the  bow  gently  across  it.  The  wire  will  vibrate  be- 
tween the  feather  and  fixed  bridge,  and  also  in  equal  divisions 
on  the  other  side  of  the  feather,  but  having  points  of  rest,  or 
nodes,  at  the  divisions.  For  example,  touch  the  feather  at  20, 
and  a  node  will  appear  at  40,  or  touch  at  12,  and  other  nodes 
will  appear  at  24,  36,  and  48,  dividing  the  wire  into  three  or  five 
equal  portions,  vibrating  at  equal  times.  Put  paper  riders,  blue 
and  red,  on  the  wire  before  sounding;  some  on  the  nodes,  and 
some  of  another  color  on  intermediate  places.  The  former  will 
remain  stationary,  while  the  latter  will  be  instantly  thrown  off. 

"  For  showing  sympathetic  vibrations,  tune  one  wire  to  unison 
(or  an  octave)  with  the  organ-pipe,  or  vibrating  plate ;  or  sound 
the  note  with  the  voice,  and  the  wire  will  be  thrown  into  vibra- 
tion and  distinctly  heard.  It  is  essential  that  the  unison  or 
chord  be  perfect,  or  the  wire  will  not  respond.  Draw  the  piston 
of  the  pipe  while  sounding  it;  the  wire  will  respond  to  the  note 
which  was  for  the  instant  of  the  same  number  of  vibrations. 

"  By  tuning  the  wires  nearly  to  unison,  the  effects  of  interfer" 
ence,  or  beats,  are  produced." 


3OO  APPENDIX. 

5.  A  violoncello  bow. 

6.  Vibrating  plate,  for  §  164. 

7.  Four  rods,  for  §  178. 

8.  Brass  rods,  for  §  179. 

9.  Iron  screw-press,  for  last  three  instruments. 

10.  Ivory  ball  and  stand,  for  use  with  No.  8. 

11.  A  resonant  jar,  for  §  180.     The  nitric-oxide  jar  (Handbook 
of  Chemistry,  page  184,  §  18)  is  just  right  for  the  purpose. 

12.  Three  glass  tubes,  for  §  182. 

13.  Organ-pipe,  with  sliding  piston." 

14.  Reed  pipe. 

15.  Jet  and  tube  for  singing  flame. 

LIGHT.  —  It  is  very  desirable  to  have  a  room  which  can  be 
darkened,  and  into  which  direct  sunlight  can  be  admitted 
through  a  hole  in  a  shutter.  The  hole  should  be  four  inches  in 
diameter,  with  a  diaphragm  for  reducing  it.  The  beam  of  light 
may  be  received  upon  a  mounted  mirror  inside  the  room,  and 
thus  thrown  wherever  desired.  It  is  better  to  introduce  the  light 
by  means  of  a  porte-lumiere.*  Some  experiments  may  be  per- 
formed by  means  of  the  light  from  a  common  solar  lamp,  having 
an  opaque  chimney  with  an  aperture  opposite  the  flame. 

1.  Condensing  lens,  mounted. 

2.  Prism  for  refraction,  etc. 

3.  Mounted  prism. 

4.  Achromatic  prism. 

5.  Apparatus  for  revolving  disks,  with  set  of  disks.     These  are 
circles  of  cardboard  and  paper  of  a  variety  of  brilliant  colors, 
including  Newton's  disk  for  recomposing  white  light.     A  great 
variety  of  interesting  and  beautiful  effects  are  produced  by  the 
set  of  disks  made  by  Ritchie,  f 

6.  Apparatus  for  Newton's  rings. 

7.  Zoetrope. 


*  See  Ritchie's  new  Catalogue  of  Apparatus. 

t  The  brief  duration  of  the  light  of  the  electric  spark  (see  §  41,  p.  258)  may  be 
shown  by  discharging  a  Leyden  jar,  so  that  the  light  produced  shall  fall  upon  one  of 
the  disks  in  rapid  rotation. 


APPENDIX.  301 

8.  Stereoscope,  with  diagrams. 

9.  Convex  and  concave  mirrors. 

10.  Mounted  mirror. 

11.  Double-convex  and  double-concave  lenses. 

HEAT.  —  i.  Iodine  cell,  for  cutting  off  luminous  radiations  in 
the  sunbeam.       , 

2.  A  differential  thermometer. 

3.  A  conductometer. 

4.  A  pair  of  tin  plates  and  balls,  for  §  257  and  §  263. 

5.  Compound  bar,  for  §  267. 

6.  Apparatus  for  convection  of  gases. 

7.  Mason's  hygrometer. 

8.  Edson's  hygrodeik. 

9.  Reflectors  and  ball. 

10.  Fire-syringe. 

11.  Spirit  lamp. 
ELECTRICITY.  —  i.  Bar  magnet. 

2.  U-magnet. 

3.  Voltaic  pair  (Figure  172). 

4.  Bunsen's  cell. 

5.  Needle  and  stand. 

6.  Dipping  needle. 

7.  Oersted's  galvanometer. 

8.  Electro-magnet  (Figure  180). 

9.  Lifting-coil  (Figure  181). 

10.  Page's  rotating  apparatus  (Figure  182). 

11.  Helix  and  ring. 

12.  Model  of  electro-magnetic  telegraph  (Figure  185). 

13.  Relay  magnet  (Figure  186). 

14.  Decomposing  cell,  §  327. 

15.  Thermo-electric  series. 

16.  Small  induction  coil. 

17.  Induction  coil,  with  separable  helices  (Figure  188). 

18.  Powder  cup. 

19.  Vulcanite  cylinder  (for  frictional  electricity). 

20.  Small  electrical  machine. 

21.  Holtz's  electrical  machine. 


302 


APPENDIX. 


Fig.  215. 


This  machine,  as  im- 
proved by  Ritchie  (Fig- 
ure 215),  consists  of  a 
thin  revolving  glass 
plate,  near  to  which,  on 
opposite  sides  of  the 
axis,  are  placed  two  (or 
four)  sectors,  or  plates 
of  thin  glass.  These 
have  pieces  of  paper 
pasted  upon  both  sides, 
covering  a  small  portion 
of  the  sectors.  On  the 
edges,  meeting  the  plate 
in  its  revolution,  a 
tongue  of  paper  pro- 
jects beyond  the  glass, 
to  collect  the  electricity 
from  the  revolving  plate.  On  the  reverse  side  of  the  rotating 
plate,  and  opposite  to  the  paper  coatings,  are  sets  of  collecting 
points,  like  those  of  the  frictional  machine.  These  are  insulated, 
but  connected  by  metal  rods  with  two  insulated  pillars,  in 
which  slide  discharging  rods  with  ball  ends. 

The  machine  is  excited  by  bringing  near  to  one  of  the  sectors, 
while  the  plate  is  rotating,  a  piece  of  vulcanite  which  has  been 
charged  by  rubbing  it  upon  a  piece  of  cat-skin.  The  plate  at 
once  shows  electrical  excitement,  which  increases  till  it  reaches 
a  maximum,  accompanied  by  a  rushing  sound ;  while  at  the 
same  time  a  perceptible  increase  of  force  is  required  to  turn  the 
plate.  Seen  in  the  dark,  the  sector  first  charged  shows  a  beau- 
tiful play  of  electrical  streamers  flowing  apparently  from  the 
plate  to  the  sector  and  more  strongly  to  the  collecting  points, 
while  the  opposite  set  of  points  show  bright  stars ;  the  one  set 
of  points  giving  negative,  and  the  other  positive  electricity.  By 
connecting  one  of  the  dischargers  with  a  prime  conductor  and 
the  other  one  with  the  earth,  the  discharge  is  intensified. 
A  very  remarkable  modification  is  made  by  connecting  both 


APPENDIX.  303 

the  sets  of  points  by  means  of  metal  conductors  with  one  prime 
conductor,  while  a  third  set  of  collecting  points  is  placed  half- 
way between  the  others,  but  without  a  sector  over  it,  and  con- 
nected with  the  earth.  As  thus  connected,  the  sectors  and 
points  continue  to  exhibit  negative  and  positive  electricity,  and 
the  prime  conductor  gives  positive  electricity  when  the  sector, 
whose  tongue  points  towards  the  third  or  "earth"  set  of  col- 
lecting points,  is  excited  by  means  of  the  vulcanite,  and  nega- 
tive electricity  when  the  opposite  sector  is  first  excited. 

No  satisfactory  explanation  of  the  action  of  the  machine 
has  yet  been  given. 

In  power  it  greatly  exceeds  the  frictional  machine.  The  quan- 
tity of  electricity  obtained  is  many  times  greater,  so  that  it 
serves  well  for  experiments  with  vacuum  tubes,  Geissler's  tubes, 
Gassiot's  cascade,  and  the  like. 

22.  Insulated  conductor,  for  §  346. 

23.  Gold-leaf  electroscope. 

24.  Ley  den  jar. 

25.  Diamond  jar. 

26.  Discharger. 

27.  Electric  wheel. 

28.  Spotted  tube  (for  electrical  light). 

29.  Stand  and  bells. 

30.  Gassiot's  cascade. 

31.  Geissler's  tube. 

THE  INDUCTORIUM,  OR  RUHMKORFF'S  COIL.  —  This  machine  is 
shown  in  Figure  216.  The  relative  position  and  construction  of 
the  primary  helix,  with  its  enclosed  core  of  iron  wire,  the  sur- 
rounding secondary  helix  of  fine  insulated  wire  of  great  length, 
and  the  rheotome,  or  break-piece,  have  been  described  in  §  336. 
In  that  instrument  (Figure  188),  the  induced  electric  current 
is  of  comparatively  low  intensity.  Hence  it  will  not  give  a 
spark  in  the  atmosphere,  though  in  a  vacuum  or  highly  rarefied 
gas  the  current  will  pass  in  a  beautiful  glow.  The  spark  or 
flash  at  the  break-piece  is  immensely  magnified  by  the  extra  cur- 
rent in  the  primary  coil.  Fizeau  found  that  by  connecting  the 


304 


APPENDIX. 


Fig-  216.  wires  on  each  side  of 

the  break  to  the  two 
coatings  of  a  Leyden 
jar,  this  extra  current 
may  be  drawn  off  and 
the  flash  or  spark  re- 
duced to  that  of  the 
direct  action  of  the 
battery,  but  the  in- 
tensity of  the  secon- 
dary current  is  enor- 
mously increased. 

In  the  inductorium 
the  Leyden  jar  is  thus 
introduced,  and  is 
made  of  many  sheets 
of  oiled  silk  coated  with  tinfoil,  which  are  equivalent  to  a  Ley- 
den jar  of  from  twenty  to  one  hundred  square  feet  of  surface. 
This  is  the  condenser,  which  is  in  the  base  of  the  instrument. 

As  the  tension  of  the  induced  current  is  increased,  we  must 
increase  the  insulation  between  the  spirals  of  the  coil  and  the 
courses  of  the  bobbin,  as  well  as  between  the  helices.  No  one 
succeeded  in  doing  this,  except  for  very  low  tensions,  until  Rit- 
chie, of  Boston,  wound  the  secondary  helix  in  planes  perpen- 
dicular to  the  axle,  thus  separating  the  portions  of  wire  under 
different  tension  so  far  that  a  discharge  cannot  take  place  be- 
tween them.  With  this  arrangement,  the  secondary  helix  may 
contain  thirty  miles  of  wire,  and  a  spark  of  fifteen  inches  or  more 
may  be  obtained. 

The  invention  of  every  part  of  the  so-called  "  Ruhmkorff's 
coil,"  except  the  condenser,  is  due  to  our  own  countrymen,  Page 
and  Ritchie.  It  was  not  until  RuhmkorfF  unwound  one  of 
Ritchie's  coils,  and  thus  learned  the  secret  of  its  structure,  that 
he  was  able  to  make  an  instrument  that  would  give  a  spark  of 
more  than  three-quarters  of  an  inch.  He  has,  however,  made 
no  acknowledgment  of  his  indebtedness  to  Ritchie. 


APPENDIX. 


305 


THE    SPECTROSCOPE  AND    THE  DIFFERENT  KINDS 
OF  SPECTRA. 

i.     The  Spectroscope.  —  This  instrument  (Figure  217)  consists 
of  a  prism,  P,  and  three  tubes,  A,  JB,  and  C.     The  tube  A  is  an 

Fig.  217. 


ordinary  telescope.  The  tube  B  has  a  narrow  slit  in  its  outer 
end,  through  which  a  beam  of  light  is  admitted.  This  beam  is 
concentrated  by  a  lens  upon  the  prism  P.  The  tube  C  has  at  its 
outer  end  a  fine  scale  marked  on  glass.  The  light  from  the 
candle  F  shines  through  this  glass,  and  is  reflected  by  the  face 
of  the  prism  into  the  telescope  A,  so  that  on  looking  into  this 
telescope  an  enlarged  image  of  the  scale  is  seen.  The  light  from 
the  tube  B,  on  passing  through  the  prism,  is  dispersed  into  a 
spectrum,  which  is  examined  by  means  of  the  telescope  A. 

This  simple  instrument  was  invented  in  1859,  ty  two  German 
professors,  Bunsen  and  Kirchhoff,  and  it  has  already  led  to  very 
remarkable  discoveries  in  both  Chemistry  and  Astronomy. 

2.  The  Spectra  of  Incandescent  Solids  and  Liquids.  —  When 
the  light  from  an  incandescent  solid  or  liquid  is  examined  by 

20 


306  APPENDIX. 

means  of  the  spectroscope,  the  spectrum  is  seen  to  be  an  un- 
broken band  of  colored  light.  Such  a  spectrum  is  called  a  con- 
tinuous spectrum.  Dense  vapors  when  incandescent  also  give 
continuous  spectra. 

3.  The  Spectra  of  Incandescent  Gases.  —  If  we  dip  a  platinum 
wire  into  a  solution  of  some  sodic  salt,  and  hold  it  in  the  color- 
less flame  of  a  Bunsen's  lamp,  so  as  to  color  it  with  incandes- 
cent sodium  vapor,  we  shall  get  a  spectrum,  consisting  of  a 
single  yellow  line,  as  shown  at  III.  in  the  chromolithic  plate  at 
the  beginning  of  this  book.    Color  the  flame  in  a  similar  manner 
with  incandescent  potassium  vapor,  and  we  get  a  spectrum  like 
II.  in  the  plate,  continuous  in  the  middle,  with  a  bright  line  at 
each  end.     The  incandescent  vapors  of  the  rare  metals  c&sium 
and  rubidium  give  spectra  like  IV.  and  V.  in  the  plate. 

As  a  rule,  the  spectra  of  incandescent  gases  are  broken,  or  made 
up  of  bright  lines  separated  by  dark  spaces.  The  spectrum  of 
each  element  is  unlike  that  of  every  other  element,  either  in  the 
number  or  the  position  of  its  bright  lines,  and  usually  in  both. 

The  position  of  these  lines  'can  be  ascertained  with  great 
accuracy  by  means  of  the  scale,  which  is  seen  in  the  telescope 
in  the  same  position  as  the  spectrum ;  and  for  the  same  sub- 
stance the  position  of  the  bright  lines  is  always  the  same. 

4.  Reversed  Spectra.  —  Kirchhoif  found   on  examining  the 
brilliant    light    from    incandescent   lime,    after    it    had    passed 
through  a  flame  colored  with  sodium  vapor,  that  its  continuous 
spectrum  was  crossed  by  a  single  black  line,  and  that  this  black 
line  was  exactly  in  the  position  of  the  bright  sodium  line.     The 
bright  yellow  line  became  a  dark  line  when  seen  against  the 
background  of  the  lime-light.      By  allowing  the  lime-light  to 
traverse  the  vapors  of  potassium,  barium,  strontium,  etc.,  the 
bright  lines  which  they  would  have  given  were  found  to  be  con- 
verted into  dark  lines. 

This  reversal  of  the  bright  lines  of  the  metallic  vapors  is 
owing  to  the  fact  that  each  vapor  has  the  power  to  absorb  rays 
having  the  same  refrangibility  as  those  it  emits,  and  no  others. 
Each  vapor,  therefore,  absorbs  from  the  lime-light  all  those 
rays  which  would  fall  in  the  part  of  the  spectrum  occupied  by 
its  own  bright  lines ;  and,  as  its  own  light  is  feeble  in  compari- 


APPENDIX. 


307 


son  with  the  lime-light,  the  bright  lines  appear  dark  in  contrast 
with  the  rest  of  the  spectrum.  The  dark  lines  produced  thus 
by  absorption  constitute  the  reversed  spectra  of  the  absorbent 
vapors. 

5.  Fraunhofer 's  Lines.  —  When  sunlight  is  examined  by 
means  of  the  spectroscope,  its  spectrum  is  found  to  be  crossed 
by  a  great  number  of  very  fine  dark  lines. 

These  dark  lines  were  first  noticed  by  Wollaston  in  1802 ;  but 
they  were  first  studied  and  described  in  detail  by  Fraunhofer  in 
1814.  Fraunhofer  mapped  the  lines,  and  designated  the  most 
conspicuous  of  them  by  the  letters  A,  a,  B,  C,  D,  E,  b,  F,  G, 
If:  they  are  therefore  generally  known  as  Fraunhofer's  lines. 

The  dark  line  A  (see  I.  of  the  chromolithic  plate  at  the  begin- 
ning of  this  book)  is  at  the  extremity,  and  B  at  the  middle,  of 
the  red ;  C  at  the  boundary  of  the  red  and  orange ;  D  in  the 
orange ;  E  in  the  green ;  F  in  the  blue  ;  G  in  the  indigo ;  and 
H  in  the  violet.  Of  the  other  prominent  lines  just  mentioned, 
a  is  in  the  red,  and  b  in  the  green.  In  the  case  of  sunlight,  the 
position  of  the  dark  lines  is  fixed  and  definite.  Similar  dark 
lines  are  found  in  the  spectra  of  starlight,  but  their  position  is 
somewhat  different.  Of  the  solar  lines,  however,  all  are  not 
invariable  in  position  and  distinctness;  some  of  the  feebler  lines 
being  seen  only  when  the  sun  is  near  the  horizon,  or  in  certain 
states  of  the  atmosphere.  The  invariable  ones  are  the  true 
Fraunhofer's  lines,  and  belong  to  the  sun.  The  others  are  sup- 
posed to  be  due  to  the  absorptive  action  of  our  atmosphere,  and 
are  therefore  called  atmospheric  or  telluric  lines. 

Fraunhofer  counted  more  than  600  dark  lines,  distributed 
irregularly  from  the  extreme  red  end  of  the  spectrum  to  the 
extreme  violet  end.  Brewster  counted  2,000.  By  causing  the 
light  to  traverse  several  prisms  in  succession,  the  number  of 
the  lines  has  been  increased  to  3,000,  and  several  which  had 
been  supposed  to  be  single  have  been  shown  to  be  double. 

Fraunhofer's  lines  are  undoubtedly  the  reversed  spectra  of 
the  incandescent  vapors  of  the  sun's  atmosphere. 

[For  the  applications  of  the  spectroscope  in  Chemistry  see  our  "Elements  of 
Chemistry,"  or  "Handbook  of  Chemistry;"  and  for  its  applications  in  Astronomy 
see  the  "  Elements  of  Astronomy,"  or  the  "  Handbook  of  the  Stirs."] 


308  APPENDIX. 

PHOTOGRAPHY. 

1.  Photography  is  the  art  of  fixing  the  image  of  the  camera 
(218)  permanently  to  the  surface  upon  which  it  falls.     This  is 
done  by  the  chemical  action  of  light. 

The  various  photographic  processes  may  be  considered  under 
three  heads :  photography  on  metal,  photography  on  paper,  and 
photography  on  glass. 

2.  Photographs  on  Metal.  —  It  was  in  the  year  1839  tnat  the 
problem  of  taking  pictures  by  light  was  first  successfully  solved 
by  a  Frenchman  named  Daguerre. 

The  Daguerreotype  picture  is  taken  on  a  plate  of  copper  coated 
with  silver.  The  plate  is  first  carefully  polished,  and  then  ren- 
dered sensitive  by  exposing  its  silvered  surface  to  the  vapor  of 
iodine,  which  forms  upon  it  a  thin  layer  of  argentic  iodide.  If 
the  picture  is  to  be  taken  quickly,  the  surface  must  be  made  still 
more  sensitive  by  the  action  of  bromine.  All  these  operations 
must  be  performed  by  candle-light.  The  plate  is  now  put  into 
a  little  wooden  case,  and  exposed  in  the  camera.  After  a  little 
time,  it  is  removed  to  a  darkened  room.  No  change  perceptible 
to  the  eye  has  taken  place ;  but  when  the  plate  is  exposed  to  the 
vapor  of  mercury,  an  image  appears  exactly  like  that  formed  in 
the  camera.  The  mercury  condenses  upon  those  parts  of  the 
plate  that  have  been  most  strongly  illumined,  and  thus  develops 
the  picture  which  before  was  latent.  The  action  of  the  light 
gives  the  molecules  of  the  argentic  iodide  a  tendency  to  decom- 
pose, and  the  tendency  of  the  mercury  to  unite  with  the  silver 
completes  the  decomposition.  In  the  shades  of  the  picture,  the 
molecules  of  the  iodide  have  acquired  no  tendency  to  break  up, 
and  those  parts  are  not  attacked  by  the  mercury. 

If,  after  the  development  of  the  picture,  the  plate  were  ex- 
posed to  the  light,  the  iodide  on  all  parts  of  the  surface  not 
attacked  by  the  mercury  would  gradually  blacken,  and  the 
picture  become  obliterated.  In  order  to  fix  the  picture,  it  is 
necessary  to  dissolve  and  remove  this  iodide,  which  is  usually 
done  by  a  solution  of  sodic  hyposulphite. 

The  picture  is  next  toned  by  immersing  the  plate  in  a  solution 
of  auric  chloride.  Some  of  the  gold  of  this  compound  unites 


APPENDIX.  309 

with  the  mercury  and  silver  of  the  parts  attacked,  an'd  greatly 
increases  the  intensity  of  the  lustre. 

The  lights  of  the  picture  consist  of  the  amalgam  of  mercury, 
silver,  and  gold ;  and  the  shades,  of  metallic  silver. 

3.  Photographs  on  Paper. — Photographs  on  paper  are  ordi- 
narily printed  from  negatives  on  glass. 

If  gun-cotton  be  put  into  a  mixture  of  alcohol  and  ether,  it 
dissolves  and  forms  collodion.  If  this  solution  is  poured  over 
any  surface,  the  alcohol  and  ether  quickly  evaporate,  leaving  a 
film  of  solid  collodion  behind. 

To  obtain  a  sensitive  surface  on  glass,  a  solution  of  collodion 
is  first  impregnated  with  potassic  iodide,  or  a  mixture  of  potassic 
iodide  and  ammonic  bromide,  and  poured  out  upon  the  surface 
of  a  glass  plate,  so  as  to  coat  it  with  a  thin  film.  The  plate 
thus  coated  is  dipped  into  a  bath  of  argentic  nitrate,  so  as  to 
form  a  film  of  argentic  iodide,  or  a  mixture  of  argentic  iodide 
and  bromide.  The  plate  is  now  exposed  a  short  time  in  the 
camera,  and  again  removed  to  a  darkened  room.  As  before,  no 
image  is  perceptible.  The  light  has  not  decomposed  the  com- 
pounds of  silver,  but  merely  given  them  a  disposition  to  de- 
compose. Their  decomposition  is  completed,  and  the  picture 
developed,  by  pouring  over  the  plate  a  solution  of  ferrous  sul- 
phate or  of  pyrogallic  acid.  The  picture  is  then  fixed  by  dis- 
solving off  the  argentic  iodide  from  the  unaffected  part  by  means 
of  the  solution  of  sodic  hyposulphite. 

The  glass  is  rendered  less  transparent  by  the  presence  of  the 
metallic  silver:  hence,  when  viewed  by  transmitted  light  the 
lights  of  the  image  appear  dark,  and  the  shades  light;  and 
the  picture  is  therefore  said  to  be  a  negative.  From  this  nega- 
tive picture  any  number  of  positive  pictures  may  be  printed  on 
paper.  For  this  purpose,  paper  is  impregnated  with  argentic 
chloride,  by  dipping  it  first  into  a  solution  of  common  salt  (sodic 
chloride),  and  then  into  a  bath  of  argentic  nitrate.  The  nega- 
tive is  then  placed  on  a  sheet  of  this  paper  in  a  copying-frame, 
and  exposed  to  the  action  of  light.  The  chloride  gradually 
blackens,  and  most  rapidly  where  the  glass  is  most  transparent. 
In  this  way  the  tints  of  the  negative  are  reversed,  and  the  picture 
becomes  a  positive.  After  sufficient  exposure,  the  picture  is 


310  APPENDIX. 

fixed  by  dissolving  off  the  remaining  chloride  by  a  solution  of 
sodic  hyposulphite,  and  toned  in  a  bath  of  auric  chloride. 

4.  Photographs  on  Glass. —  The  picture  on  glass,  which  ap- 
pears negative  by  transmitted  light,  will  become  positive  if  it  be 
backed  with  a  coating  of  black  varnish  or  a  piece  of  black  cloth, 
so  that  it  shall  be  seen  by  reflected  light. 

Beautiful  positive  pictures  on  glass  may  be  obtained  by  the 
following  process :  prepare  the  plate  in  the  same  way  as  for 
negatives,  but  expose  it  a  much  shorter  time  in  the  camera; 
develop  the  picture  by  pouring  over  it  a  solution  of  ferrous  sul- 
phate, which  gives  a  negative  image ;  then  pour  a  solution  of 
potassic  cyanide  over  the  plate,  and  this  negative  is  rapidly 
converted  into  a  positive. 


NOTES. 


B3^  These  notes  are  numbered  to  correspond  with  the  sections  to  which  they  refer. 

257.  Many  familiar  illustrations  of  the  fact  that  good  absorbers 
are  good  radiators,  and  vice  versa,  might  be  given.  Put  equal 
quantities  of  boiling  water  into  two  teakettles,  one  of  which  is 
polished  and  the  other  rough,  and  the  former  will  cool  more 
slowly  than  the  latter.  Put  the  same  kettles  full  of  cold  water 
before  an  open  fire,  and  the  rough  one  will  become  hot  sooner 
than  the  other. 

Dark  colors  absorb  and  radiate  heat  better  than  light  ones. 
The  former  are  therefore  the  better  for  winter  clothing,  and  the 
latter  for  summer.  For  a  similar  reason,  snow  melts  very  slowly 
even  under  the  direct  rays  of  the  sun ;  but  a  piece  of  black  cloth 
laid  upon  the  snow  causes  it  to  melt  quite  rapidly. 

Since  a  stove  is  meant  to  radiate  heat,  it  is  better  that  its  sur- 
face should  be  rough  than  that  it  should  be  polished.  On  the 
other  hand,  a  tea-urn,  or  any  vessel  intended  to  keep  its  contents 
hot  as  long  as  possible,  should  be  polished  rather  than  rough. 


APPENDIX.  311 

276.  A  still  lower  temperature,  of — 220°,  has  been  obtained 
bj  placing  a  mixture  of  liquid  nitrous  oxide  and  carbonic  di- 
sulphide  (bisulphide  of  carbon)  in  an  exhausted  receiver. 

Water  may  be  readily  frozen  by  the  evaporation  of  ether. 
Put  the  water  in  a  small  test-tube ;  and  place  the  test-tube, 
surrounded  with  cotton  moistened  with  ether,  in  a  wine-glass 
or  tumbler.  Put  the  nozzle  of  a  bellows  into  the  cotton,  and 
blow  vigorously.  The  current  of  air  passing  over  the  cotton 
acts  on  a  very  large  surface  of  ether,  which  is  thus  evaporated 
fast  enough  to  freeze  the  water  in  the  tube. 

277.  The  force  exerted  in  the  expansion  and  contraction  of 
bodies  is  very  great.     A  curious  application  of  this  force  was 
made  by  the  architect  Molard,  at  the  Conservatoire  des  Arts  et 
Metiers,  in  Paris.     The  walls  of  a  vaulted  gallery  in  this  build- 
ing had  been  pushed  outward  by  the  weight  of  the  stone  roof, 
and  it  was  feared  that  the  whole  would  fall.     Molard  put  iron 
bars  across  the  gallery  through  the  walls,  the  ends  of  the  bars 
having  a  screw-thread  fitted  with  nuts.     He  then  heated  the 
bars  throughout  their  whole  length,  screwed  them  up  tight,  and 
allowed  them  to  cool.    The  gradual  contraction  of  the  iron  drew 
the  walls  nearer  together  without  injuring  them.     The  process 
was  repeated  several  times,  until  the  walls  were  restored  to  a 
vertical  position.    The  bars  were  left  to  keep  them  in  place,  and 
may  be  seen  to  this  day. 

Advantage  is  taken  of  the  force  of  contraction  in  putting  tires 
on  wheels.  The  tire  is  put  on  hot,  when  it  fits  loosely;  but  as  it 
cools  it  contracts,  and  grasps  the  wheel  with  very  great  force. 

For  other  illustrations  of  the  kind,  see  §  295. 

287.  The  vapor  in  the  atmosphere  acts  in  the  same  way  as  the 
glass  of  the  hot-house :  the  luminous  rays  from  the  sun  easily 
penetrate  it,  and  fall  upon  the  earth ;  but  they  cannot  make  their 
way  back  through  it  when  radiated  from  the  earth  as  obscure 
heat.  See  the  chapter  on  the  Physics  of  the  Atmosphere,  §  2. 

Saussure  made  a  wooden  box,  blackened  within,  having  one 
of  its  sides  formed  of  three  panes  of  glass,  separated  by  thin 
layers  of  air.  He  then  put  a  vessel  of  water  in  the  box,  and 


312  APPENDIX. 

exposed  the  glass  side  to  the  rays  of  the  sun ;  and  in  this  way 
he  succeeded  in  making  the  water  boil.  The  luminous  heat 
easily  passed  in  through  the  glass  and  the  air,  and  was  absorbed 
by  the  blackened  surface ;  but  when  radiated  back  as  obscure 
heat,  it  could  not  escape  from  the  box,  and  after  a  time  it  had 
accumulated  sufficiently  to  boil  the  water. 

307.  The  zinc  used  for  battery  purposes  should,  in  all  cases, 
be  amalgamated.  Full  directions  for  the  process  are  given  in 
the  Natural  Philosophy,  page  379. 

312.  To  make  a  simple  rheotome  and  rheotrope,  fill  two  small 
cups  with  mercury,  and  put  the  ends  of  the  battery  wires  into 
them.  Put  the  end  of  another  wire  into  each  cup,  and  use  these 
latter  wires  to  convey  the  current  where  you  wish  to  use  it.  The 
current  can  be  instantly  broken  by  taking  one  of  these  wires  out 
of  the  mercury;  and  the  direction  of  the  current  can  be  changed 
by  shifting  the  wires  from  one  cup  to  the  other. 

For  a  description  of  Foucault's  self-acting  rheotome,  see 
Natural  Philosophy,  Part  II.,  page  301. 

333-  Various  arrangements  have  been  invented  for  giving 
steadiness  to  the  electric  light  by  keeping  the  carbon  points 
within  such  a  distance  of  each  other  that  the  current  can  pass 
between  them.  Foucault,  aided  by  Duboscq,  was  the  first  (in 
1849)  to  construct  an  electric  lamp  of  this  kind.  In  it,  by  means 
of  an  electro-magnet  and  of  clock-work,  the  points  are  made  to 
travel  towards  each  other  at  rates  corresponding  to  those  of 
their  combustion,  the  positive  pole  moving  faster  than  the 
negative. 

The  electric  lamp  has  not  yet  been  used  successfully  for  light- 
ing streets.  The  light  may  be  kept  up  for  hours,  but  even  then 
it  is  not  perfectly  steady,  and  the  apparatus  cannot  be  safely  left 
without  an  attendant.  It  has,  however,  been  used  with  excellent 
effect  where  a  limited  space  had  to  be  lighted  for  a  few  nights,  as 
in  building  bridges.  It  has  also  been  used  with  success  for  light- 
houses, in  England  and  France.  The  power  of  the  electric 
light  to  penetrate  fogs  is  found  to  be  far  superior  to  that  of  the 
usual  oil  light. 


APPENDIX.  313 

When  an  electric  current  is  made  to  pass  through  highly 
rarefied  air,  a  very  beautiful  effect  is  produced.  This  may  be 
shown  by  Getssler's  tubes  (so  called  from  the  inventor),  which 
are  combinations  of  bulbs  and  tubes,  filled  with  rarefied  gases 
and  liquids,  and  then  sealed  air-tight,  so  as  to  be  ready  for  use 
at  any  time.  One  of  them  is  represented  in  Figure  218.  When 

Fig.  218. 


'the  current  is  sent  through  these  tubes,  they  exhibit  lights  of 
various  tints  according  to  the  gases  contained  in  them. 

A  very  pleasing  illustration  of  the  electric  light  in  rarefied  air 
is  afforded  by  the  "  guinea  and  feather  tube,"  shown  in  Figure 
50.  If  the  ends  of  the  tube  are  connected  with  the  poles  of  the 
inductorium,  or  with  the  electrical  machine,  purple  flashes  of 
auroral  light  mark  the  passage  of  the  current  through  the  tube 
when  the  air  is  exhausted.  In  all  experiments  of  this  kind,  the 
room  should  be  darkened. 

Gassiofs  cascade  is  a  simple  and  inexpensive  piece  of  apparatus 
for  showing  the  electric  light  in  a  vacuum.  It  consists  of  a 
large  glass  goblet  (uranium  glass  is  best),  the  inside  of  which  is 
coated  nearly  to  the  top  with  tinfoil.  Place  the  vessel  on  the 
plate  of  the  air-pump,  cover  it  with  a  receiver  which  has  a 
sliding  rod  through  the  top,  bring  the  sliding  rod  in  contact 
with  the  tinfoil  coating,  and  connect  one  pole  of  the  induc- 
torium (or  one  conductor  of  the  electrical  machine)  with  the 
rod,  and  the  other  with  the  pump-plate.  When  the  air  is  ex- 
hausted, and  the  current  sent  through  the  receiver,  streams  of 
blue  light  flow  from  the  tinfoil  over  the  side  of  the  vessel  to  the 
pump-plate.  A  variety  of  beautiful  effects  are  produced  by 
different  degrees  of  exhaustion,  and  by  changing  the  direction 
of  the  current. 

The  apparatus  known  as  the  Abbt  Nollefs  Globe  also  furnishes 
very  pretty  displays  of  the  electric  light  in  rarefied  air.  It  con- 
sists of  a  glass  globe  suspended  in  the  upper  part  of  a  glass  bell- 
jar,  and  arranged  so  that  it  can  be  partially  filled  with  water, 
and  then  connected  with  the  inductorium  or  the  electrical  ma- 


314  APPENDIX. 

chine  by  a  chain  dipping  into  the  water.  The  light  in  this  case 
flows  in  lambent  streams  from  the  globe  to  the  pump-plate. 

A  variety  of  pieces  of  apparatus  for  showing  the  electric 
light  are  made  by  pasting  bits  of  tinfoil,  about  -^  of  an  inch 
apart,  on  glass,  oiled  silk,  or  other  non-conducting  substance. 
Letters,  outline  figures,  etc.,  may  thus  be  formed,  which  appear 
in  lines  of  scintillating  light  when  the  current  is  sent  through 
them. 

The  pieces  of  tinfoil  may  be  pasted  in  a  spiral  on  the  inside 
of  a  long  glass  tube,  and  lighted  up  in  the  same  way. 

The  diamond  jar,  as  it  is  sometimes  called,  is  a  Leyden  jar,- 
the  coatings  of  which  are  composed  of  small  pieces  of  tinfoil, 
separated  from  one  another.  Brilliant  sparks  pass  between 
these  pieces  when  the  jar  is  charged  or  discharged. 

336.  A  Ruhmkorff  's  coil  of  moderate  size  readily  yields  sparks 
of  from  four  to  five  inches,  with  a  battery  of  six  Bunsen's  cells. 
The  power  of  the  induced  current  to  turn  a  needle,  and  to  effect 
electrolysis,  is  very  slight.  This  shows  that  it  is  very  much  in- 
ferior to  the  inducing  current  in  quantity,  though  muck  superior 
in  tension.  The  physiological  effect  is  very  powerful,  and  care 
must  be  taken  not  to  allow  any  part  of  the  body  to  form  the  con- 
nection between  the  poles,  as  the  shock  might  be  disagreeable, 
if  not  dangerous. 

347.  When  an  insulated  conductor  is  brought  near  a  charged 
body,  it  is  first  polarized ;  and  the  nearer  it  is  brought,  the 
higher  the  polarization  rises.  If  the  conductor  discharges  its 
force  at  the  end  nearest  the  polarizing  body,  it  becomes  charged 
with  the  same  electric  force  as  the  polarizing  body;  if  it  dis- 
charges from  the  opposite  end,  it  becomes  charged  with  the  force 
opposite  to  that  on  the  polarizing  body.  If  the  conductor  can 
discharge  quite  readily  at  both  ends,  but  more  readily  at  one  end 
than  at  the  other*  there  will  be  three  steps  in  the  process.  It  will 
first  become  polarized,  then  charged,  and  finally  neutralized. 

If  the  conductor  can  discharge  quite  readily,  and  with  equal 
readiness  at  each  end,  there  will  be  only  two  steps  in  the  process  : 
it  will  be  first  polarized,  and  then  neutralized. 


QUESTIONS   FOR   REVIEW   AND 
EXAMINATION. 


GSF*  The  numbers  refer  to  the  sections  of  the  book. 

COHESION.  —  i.  Show  that  bodies  are  made  up  of  molecules. 
What  are  molecules?  2.  Show  that  molecules  are  very  small. 
3.  State  the  effect  of  cold  upon  solids,  liquids,  and  gases.  What 
follows  from  this  ?  4.  What  is  true  of  the  spaces  between  mole- 
cules? 5.  What  forces  act  between  molecules?  Prove  this. 
6.  Show  that  these  forces  act  together.  7.  Define  the  three 
states  of  matter.  8.  What  is  cohesion  ?  adhesion  ?  9.  Through 
what  distances  do  these  forces  act?  Prove  this.  10.  What  is 
true  of  cohesion  in  solids?  n.  Define  tenacity.  Describe  the 
dynamometer,  and  explain  its  use.  12.  When  is  a  solid  hard? 
When  soft?  What  is  the  test  of  hardness  ?  13.  Define  and  illus- 
trate elasticity.  What  is  meant  by  the  limit  of  elasticity?  When 
is  a  body  brittle?  When  malleable?  When  ductile?  How  is 
gold-leaf  made?  Wire?  What  facts  about  iron  wire ?  14.  Show 
that  solids  are  compressible.  15.  What  are  crystals?  How  may 
we  get  crystals  of  alum?  of  sulphur?  In  order  to  crystallize, 
in  what  state  must  the  substance  be?  Why?  How  are  large 
crystals  obtained?  Explain  the  formation  of  crystals  in  iron 
axles,  etc.  What  is  said  of  ice  ?  16.  What  is  true  of  the  differ- 
ent sides  of  molecules?  Prove  this.  17.  Describe  Rupert's 
drops.  Explain  annealing  and  tempering.  18.  What  is  true  of 
cohesion  in  liquids?  Of  the  spaces  between  the  molecules  in 
liquids?  19.  Are  liquids  compressible?  How  may  this  be 
proved?  20.  Are  liquids  elastic?  Show  this.  21.  How  do  the 
molecules  of  liquids  tend  to  arrange  themselves?  Give  illustra- 
tions. 22.  What  is  said  of  cohesion  in  gases?  Of  the  molecules 
of  gases?  23.  What  of  the  compressibility  of  gases?  24.  What 
of  their  elasticity?  Recite  the  Summary  of  Cohesion. 


316  APPENDIX. 

ADHESION.  —  25.  Give  illustrations  of  adhesion  between  solids. 
What  is  sometimes  true  of  this  adhesion?  26.  What  is  said  of 
adhesion  between  solids  and  liquids?  27.  Describe  the  experi- 
ment with  balance  and  glass  disk.  What  does  it  show?  28. 
What  is  shown  by  the  experiment  with  a  glass  plate  laid  on 
water?  What  is  the  effect  of  pulverizing  a  solid?  Why?  Ex- 
plain the  clarifying  of  liquids.  29.  What  is  shown  by  the 
experiment  with  Epsom  salts?  30.  State  the  three  cases  of 
adhesion  between  solids  and  liquids.  31.  How  does  heat  affect 
solution?  Why?  32.  What  is  capillarity?  The  origin  of  the 
word?  State  the  different  cases  of  capillarity.  33.  Give  illus- 
trations of  capillarity.  34.  Show  the  strength  of  capillarity. 
35.  Will  a  liquid  overflow  a  capillary  tube?  Show  this.  Ex- 
plain the  burning  of  a  common  lamp.  Why  must  an  alcohol 
lamp  have  a  cap?  36.  Describe  the  experiment  with  ammonia 
and  charcoal.  What  does  it  show?  Why  is  the  charcoal  first 
heated?  37.  Give  the  facts  concerning  the  adhesion  of  liquids 
to  liquids.  38.  What  is  the  diffusion  of  liquids?  Illustrate  by 
experiment.  39.  Describe  and  illustrate  osmose  of  liquids.  40. 
What  experiment  shows  the  adhesion  of  liquids  and  gases?  41. 
Illustrate  the  effect  of  cold  and  of  pressure  on  this  kind  of  ab- 
sorption. What  is  said  of  aqua  ammonia?  of  spring-water? 
42.  Describe  and  illustrate  diffusion  of  gases.  43.  What  is 
osmose  of  gases?  Illustrate.  Recite  the  Summary  in  full. 

PRESSURE. — 44.  What  is  gravity?  What  law  of  gravity  is 
mentioned?  45.  What  is  weight?  46.  Describe  the  spring 
balance.  47.  Describe  the  balance.  48.  The  steelyard.  49. 
Define  centre  of  gravity.  56.  What  is  sometimes  true  of  the 
position  of  this  centre?  Illustrate.  51.  Define  equilibrium.  Its 
kinds?  52.  Show  that  the  centre  of  gravity  seeks  the  lowest 
point.  On  what  does  stability  of  equilibrium  depend?  Prove 
this.  What  does  the  experiment  with  the  cork  balanced  on  a 
needle  show?  What  other  illustrations  of  the  same  kind?  53. 
How  do  we  find  the  centre  of  gravity  of  a  solid?  54.  How  is  the 
weight  of  a  liquid  found  ?  55.  How  do  liquids  press  when  acted 
on  by  gravity?  56.  What  is  true  of  the  pressure  while  the  depth 
of  the  liquid  remains  the  same?  Erove  this.'  57.  At  varying 


APPENDIX. 

depths,  what  is  true  of  the  pressures?  Prove  this.  58.  Explain 
the  effect  of  additional  pressure  exerted  on  any  particle  of  a 
liquid  in  a  closed  vessel.  59.  Describe  the  hydrostatic  press. 
Explain  its  working.  Its  uses?  60.  What  is  said  of  springs? 
Of  Artesian  wells?  61.  Show  that  a  body  is  buoyed  up  by  a 
liquid.  62.  How  much  is  it  thus  buoyed  up?  Prove  this.  When 
will  a  body  float  in  a  liquid?  How  can  iron  be  made  to  float  on 
water?  63.  What  is  true  of  the  density  of  bodies?  What  is  the 
specific  gravity  of  a  body?  64.  How  do  we  find  the  specific 
gravity  of  solids?  65.  Describe  the  two  forms  of  hydrometer. 
Their  use?  Give  other  ways  of  finding  the  specific  gravity  of 
liquids.  66.  Prove  that  gases  have  weight.  67.  What  is  true 
of  the  pressure  of  gases  ?  Show  this  by  experiments.  68.  Show 
that  gases  have  an  expansive  force.  69.  Describe  the  air-pump, 
and  explain  its  action?  70.  Prove  that  bodies  are  buoyed  up  in 
air.  How  much?  71.  Why  do  balloons  rise?  How  are  they 
made?  72.  Show  that  the  pressure  of  the  air  will  sustain  a 
column  of  liquid  in  an  inverted  vessel.  73.  How  high  a  column 
of  mercury  will  it  sustain  ?  74.  How  much  is  the  pressure  of  the 
air  on  a  square  inch  ?  Show  this.  75.  Is  the  pressure  always 
the  same?  76.  How  is  the  pressure  affected  by  the  height  of  the 
place?  77.  What  is  a  barometer?  Describe  the  form  given  here. 
78.  What  are  the  uses  of  the  barometer?  79.  Describe  and  ex- 
plain the  lifting-pump.  The  different  forms  of  force  pump.  The 
fire-engine.  80.  What  is  a  siphon?  Explain  its  action.  81. 
Describe  Tantalus's  cup.  What  is  said  of  certain  springs?  82. 
What  does  the  air-gun  illustrate?  Describe  it.  Explain  the 
action  of  gunpowder  on  a  bullet.  Describe  the  condenser.  83. 
State  Mariotte's  law.  Why  is  it  so  called?  84.  Describe  the 
spirit  level. 

MOTION. — 85.  Define  inertia.  86.  What  is  the  first  law  of 
motion  ?  87.  What  is  necessary  to  make  a  body  move,  or  to 
change  the  rate  of  its  motion  ?  Show  this  to  be  so.  88.  What 
is  the  effect  of  a  force  acting  for  a  moment?  89.  Of  a  force  act- 
ing continuously?  90.  What  is  true  of  the  resistance  a  moving 
body  meets  ?  91.  When  is  a  moving  body  in  equilibrium  ?  Illus- 
trate. 92.  What  is  the  second  law  of  motion?  Illustrate  it. 


318  APPENDIX. 

93.  When  does  a  body  move  in  a  straight  line?  a  curved  line? 

94.  How  does  gravity  tend  to  make  all  bodies  fall?     Prove  this 
by  an  experiment.    95.  How  does  gravity  affect  the  velocity  of 
a  body  moving  downward?    96.  How  far  will  a  body  fall  in  a 
given  time?    97.  What  is  the  effect  of  gravity  on  a  body  moving 
upward  ?     98.  How  high  will  such  a  body  rise  in  a  given  time  ? 
99.  Define  mass  and  momentum.      100.  What  is  the  third  law  of 
motion  ?    What  is  it  often  called  ?    Give  illustrations  of  the  law. 
101.  Show  that  it  takes  time  to  give  motion  to  a  body  as  a  whole. 
On  what  does  the  piercing  power  of  a  projectile  depend?     102. 
What  is  reflected  motion ?     Its  law?     103.  What  is  a  pendulum? 
104.  What  is  the  first  law  of  the  pendulum?     Explain  the  word 
isochronis'm.     105.  The  second  law  of  the  pendulum  ?    106.  The 
third  law?      107.  The  fourth  law?      108.  The  chief  use  of  the 
pendulum?    What   is   a   clock?    Describe   its   parts,  and   their 
working. 

MACHINES.  — 109.  What  is  a  lever?  The  weight?  The  power? 
The  fulcrum?  The  arms  of  the  lever?  The  three  kinds?  no. 
What  is  the  law  of  the  lever?  Illustrate,  in.  The  general  law 
of  machines?  When  does  there  seem  to  be  a  gain  of  power  in  a 
machine?  When  a  loss  of  power?  112.  Explain  the  gain  and 
loss  of  power  in  the  three  kinds  of  lever.  113.  Describe  and 
explain  the  compound  lever.  114.  What  is  said  of  bent  levers? 
115.  Describe  the  rack  and  pinion.  116.  Of  what  is  it  a  modifi- 
cation? Show  this.  117.  Describe  the  windlass.  118.  The 
capstan.  119.  The  wheel  and  axle.  Show  the  application  of 
the  general  law  to  this  machine.  120.  Describe  the  ratchet.  Its 
use?  121.  What  is  wheel-work?  Why  used?  Describe  the  dif- 
ferent kinds  of  wheels.  How  are  they  made  to  act  on  one 
another?  122.  For  what  is  the  pulley  used?  123.  Define  fixed 
and  movable  pulleys.  124.  The  law  of  the  pulley?  125.  Show 
the  application  of  the  law  of  machines  to  systems  of  pulleys 
with  one  rope.  126.  Describe  systems  with  more  than  one  rope. 
127.  What  is  an  inclined  plane?  128.  The  law  of  the  inclined 
plane?  129.  What  is  a  wedge?  Its  law?  130.  Its  uses?  131. 
What  is  a  screw?  Its  parts?  132.  Describe  the  endless  screw. 
133.  What  kinds  of  water-wheels?  134.  Describe  the  breast 


APPENDIX.  3 19 

wheel.  The  overshot  wheel.  The  undershot  wheel.  135. 
Describe  Barker's  mill.  Explain  its  action.  136.  What  is  said 
of  the  turbine?  137.  Show  how  steam  maybe  made  to  move 
a  piston.  What  is  reciprocating  motion?  How  changed  to 
rotary  motion  ?  Describe  the  engine  in  Figure  86.  138.  De- 
scribe the  governor.  Its  use  ?  139.  What  is  the  fly-wheel  ?  Its 
use?  149.  What  is  a  high-pressure  engine?  A  low-pressure 
engine?  141.  The  purpose  of  the  boiler?  Its  construction? 
Describe  the  Cornish  boiler.  The  locomotive  boiler.  142.  De- 
scribe the  other  parts  of  the  locomotive. 

SOUND.  — 143.  Show  that  a  sounding  body  vibrates.  144. 
Show  that  sound  does  not  pass  in  a  vacuum.  145.  Show  that 
it  passes  through  all  gases.  146.  Does  sound  pass  through 
liquids?  Solids?  Prove  this.  How  is  sound  produced?  147. 
On  what  does  its  intensity  depend  ?  Show  this.  148.  The  law 
of  the  intensity  of  sound  for  different  distances?  149.  Explain 
speaking-tubes.  150.  The  velocity  of  sound  in  air?  How  found? 
151.  Its  velocity  in  water?  When  and  how  found?  152.  Its 
velocity  in  solids?  153.  When  is  sound  reflected?  The  law  of 
the  reflection ?  154.  What  are  echoes?  Mention  some  remark- 
able echoes?  155.  What  is  noise?  musical  sound?  Describe 
experiment.  156.  On  what  does  the  pitch  of  musical  sounds 
depend?  157.  Describe  the  tuning-fork.  158.  Describe  the 
siren.  159.  Its  use?  160.  What  is  an  octave?  161.  Describe 
the  sonometer.  Its  use?  162.  On  what  does  the  rapidity  of  the 
vibration  of  a  string  depend?  163.  What  are  notes?  Give  ex- 
periments illustrating  their  formation  in  strings.  164.  How  may 
nodes  be  formed  in  plates  ?  What  are  nodal  lines  ?  165.  What 
are  overtones?  166.  What  is  quality  in  sound?  Illustrate. 
167.  Show  the  transmission  of  musical  sounds  through  liquids 
and  solids.  168.  What  are  sympathetic  vibrations?  Illustrate. 
169.  Illustrate  the  interference  of  sounds.  170.  When  are  beats 
produced?  171.  What  is  unison?  A  fifth?  A  fourth?  Are  they 
equally  pleasing  to  the  ear?  What  is  a  major  third?  A  minor 
third?  A  chord?  A  discord?  172.  What  are  stringed  instru- 
ments? 173.  What  is  the  use  of  sounding-boards?  174-177. 
State  the  laws  of  the  vibration  of  strings.  What  two  classes 


32O  APPENDIX. 

of  stringed  instruments?  178.  What  is  said  of  longitudinal 
vibrations  in  rods  free  at  one  end?  179.  In  rods  free  at  both 
ends?  180.  Give  experiment  illustrating  resonance?  181.  How 
may  a  column  of  air  in  a  tube  be  made  to  vibrate?  182.  At 
what  rate  does  such  a  column  vibrate?  183.  What  is  true  of  the 
vibration  in  open  and  in  stopped  tubes  ?  184.  What  are  organ- 
pipes?  Explain  their  action.  185.  What  is  a  reed?  How  does 
it  produce  sound?  186.  What  two  classes  of  wind  instruments? 
187.  Show  that  friction  is  always  rhythmic.  How  may  the  noise 
of  a  flame  be  changed  to  music?  188.  What  is  said  of  sensitive 
flames  within  tubes?  189.  Describe  the  organ  of  voice  in  man. 
How  may  its  action  be  illustrated?  190.  Describe  the  human 
ear?  191.  What  is  the  range  of  human  hearing? 

LIGHT.  —  192.  What  is  a  luminous  body?  What  is  true  of  it? 
What  are  transparent  bodies?  opaque  bodies?  193.  Show  that 
light  traverses  space  in  straight  lines.  Explain  shadows.  De- 
fine ray,  beam,  pencil,  divergent  pencil,  and  convergent  pencil. 
194.  What  is  the  velocity  of  light?  How  first  determined?  195. 
How  does  the  intensity  of  light  vary  with  the  distance?  196. 
When  is  light  said  to  be  reflected  ?  What  is  a  prism  ?  When  is 
light  refracted  ?  197.  What  is  the  law  of  reflection  ?  198.  What 
is  diffused  light?  How  distinguished  from  reflected  light?  199 
What  is  the  law  of  refraction  ?  200.  When  is  light  totally  re- 
flected? 201.  Describe  and  explain  mirage.  202.  What  is  true 
of  rays  passing  through  a  medium  with  parallel  faces  ?  203. 
What  of  the  path  of  rays  through  a  prism?  204.  What  is  meant 
by  the  solar  spectrum?  What  are  its  colors  called?  Define 
dispersion  and  dispersive  power.  205.  Describe  and  explain  the 
achromatic  prism.  206.  Show  that  prismatic  colors  are  simple. 
207.  Show  that  they  are  unequally  refrangible.  208.  What  is 
the  composition  of  white  light?  How  may  this  be  shown  ?  How 
many  simple  colors  are  there?  What  are  they?  What  are  com- 
plementary colors?  209.  What  is  said  of  the  absorption  of 
light?  210.  To  what  is  the  color  of  bodies  due?  What  is  some- 
times true  of  the  color  they  transmit?  211.  Describe  Newton's 
rings.  What  causes  them?  212.  What  are  diffraction  fringes? 
Their  cause?  213.  What  is  double  refraction?  214.  Describe 


APPENDIX.  321 

the  polarization  of  light?  215.  What  are  lenses?  Their  differ- 
ent forms?  216.  How  do  convex  lenses  affect  parallel  rays? 
How  do  concave  lenses?  What  are  they  respectively  called? 
217.  How  may  images  be  formed  by  convex  lenses?  On  what 
does  the  size  and  position  of  the  image  depend?  218.  Describe 
the  camera  obscura?  219.  Describe  the  parts  of  the  eye.  What 
purpose  does  the  iris  serve?  220.  Show  that  the  eye  must  adjust 
itself  for  different  distances.  How  does  it  do  this  ?  221.  Describe 
the  structure  of  the  retina.  222.  Show  that  the  optic  nerve  is 
blind.  223.  How  may  the  sensation  of  light  be  excited?  224. 
How  long  does  the  impression  on  the  retina  last?  Prove  this. 
Describe  the  zoetrope.  225.  Explain  and  illustrate  irradiation. 

226.  Show  that  the  sensibility  of  the  retina  is  soon  exhausted. 

227.  What  is  color-blindness?     228.  What  is  the  optical  axis? 
the  visual  angle?    229.  How  do  we  estimate  the  size  of  bodies? 
230.  Their  distance?     231.  Why  do  near  bodies   appear  solid? 
232.  Describe  and  explain  the  stereoscope.     233.  What  are  the 
laws  of  distinct  vision?    Explain  near-sightedness  and  far-sight- 
edness.    How  may  these  defects  be  remedied  ?     234.  How  is  the 
eye  affected  by  age  ?    How  may  this  defect  be  remedied  ?    235. 
What  is  a  microscope?    A  simple  microscope?    236.  Describe 
the  compound  microscope.     What  is  aberration  ?     How  may  it 
be  lessened?     How  is  the  magnifying  power  of  a  microscope 
estimated?    237.  What  is  a  telescope ?    How  constructed?    How 
does  it  differ  from  the  microscope  ?     How  can  a  lens  be  rendered 
achromatic?     238.  Describe  the  terrestrial  telescope.     239.  The 
opera-glass.     240.  The  magic  lantern.     241.  What  is  a  mirror? 
A  plane  mirror?     How  does  it  affect  the  rays  falling  on  it  from 
an  object?     Why  is  no  image  formed?     242.  What  is  a  concave 
mirror?      How  does  it   affect  parallel  rays?  other  rays?-    243. 
When  is  an  image  of  an  object  formed  by  a  concave  mirror? 
On  what  does  its  size  depend?      244.  What  is  a  convex  mirror? 
Its  effect  on  different  kinds  of  rays?    245.  Describe  the  reflecting 
telescope.     What  is  a  refracting  telescope?    246.  How  does  a 
parabolic  mirror  affect  parallel  rays?     How  does  it  affect  rays 
diverging  from  the  focus  ?    Why  is  this  ? 

HEAT.  —  247.  How  is  heat  radiated  ?    248.  What  is  the  velocity 

21 


322  APPENDIX. 

of  radiant  heat?  249.  Define  luminous  and  obscure  heat.  250. 
When  is  a  body  diathermanous  ?  Illustrate.  251.  Show  that  ob- 
scure always  accompanies  luminous  heat.  252.  Show  that  heat 
may  be  reflected  and  refracted.  253.  How  do  the  two  kinds  of 
heat  compare  in  respect  to  refrangibility?  254.  Of  what  is  the 
spectrum  made  up?  What  are  Fraunhofer's  lines?  255.  What 
is  calorescence  ?  Fluorescence  ?  256.  What  facts  are  given  con- 
cerning the  absorption  of  heat?  257.  Show  that  good  absorbers 
are  good  radiators.  258.  What  is  conduction?  259.  What  is 
true  of  the  conductivity  of  solids?  260.  Of  liquids?  of  gases? 
261.  What  is  the  first  effect  of  heat  on  bodies?  262.  How  much 
heat  does  a  body  give  out  in  cooling  i°?  Show  this.  263.  What 
is  true  of  the  heat  required  to  raise  the  temperature  of  different 
bodies  i°?  264.  What  is  a  unit  of  heat?  265.  Define  specific 
heat.  266.  What  is  the  second  effect  of  heat  on  bodies?  267. 
What  is  said  of  the  melting-points  of  different  solids?  What 
change  do  some  bodies  undergo  before  melting?  Illustrate. 
268.  What  is  the  latent  heat  of  a  liquid?  269.  What  is  said  of 
the  boiling  of  liquids?  270.  Show  that  gases  have  latent  heat. 

271.  Show  the  relation  of  the  state  of  a  body  to  its  temperature. 

272.  Is  the  boiling-point  of  water  always  the  same?    Why  is 
this  ?    273.  Explain  the  spheroidal  state.     274.  What  is  said  of 
evaporation?    275.  At  what  point  does  a  gas  condense?     276. 
On  what  principle   do   freezing-mixtures   depend?      Illustrate. 
277-279.  What  is  true  of  the  expansion  of  solids  by  heat?     Of 
liquids?    Of  gases?    280.  What  is  convection?      281.  How  may 
convection  in  liquids  be  shown  ?     282.  How  are  oceanic  currents 
caused  ?     283.  Illustrate  the  convection  of  gases.     Explain  heat- 
ing by  a  furnace.     284.  State  the  relation  of  water  to  heat  and 
climate.     285.  What  is  peculiar  in  the  expansion  and  contraction 
of  water?    Is  the  fact  of  any  importance  ?    Why?     286.  Explain 
heating  by  steam  ?    287.  Show  that  a  hot-house  is  a  trap  for 
sunbeams.     288.  Describe  the  making  and  graduating  of  a  com- 
mon thermometer.     Explain  the  Fahrenheit,  Centigrade,   and 
Reaumur's  scales.     289.  When  is  an  alcohol  thermometer  used? 
Why?     290.  Describe  the  air  thermometer.      291.  Describe  Les- 
lie's differential  thermometer.     292.  How  are  clocks  and  watches 


APPENDIX.  323 

affected  by  temperature?  293.  Describe  Graham's  pendulum. 
294.  Describe  the  compensation  balance-wheel.  295.  Give  illus- 
trations of  the  force  exerted  by  bodies  in  expanding  and  con- 
tracting. 296.  Describe  Mason's  hygrometer.  297.  What  is  the 
hygrodeik  ? 

ELECTRICITY.  —  298.  What  is  magnetism?  The  origin  of  the 
name?  What  are  lodestones?  Why  so  called?  299.  Where 
does  the  power  of  a  magnet  chiefly  reside?  Show  this.  What 
are  magnetic  curves?  300.  What  are  the  poles  of  a  magnet? 
What  is  true  of  the  forces  at  the  poles?  301.  Describe  the  mag- 
netic needle.  302.  Show  that  the  earth  acts  like  a  magnet.  303. 
What  is  the  law  of  magnetic  attraction  and  repulsion?  304. 
Describe  and  illustrate  magnetic  induction.  305.  What  are  the 
kinds  of  magnets?  306.  Describe  the  voltaic  pair,  giving  the 
names  of  its  parts.  What  is  electricity?  The  electric  current? 
Why  so  called?  307.  Describe  Bunsen's  cell.  Grove's  cell. 
308.  Daniell's  cell.  309.  What  is  an  electric  battery  ?  What  two 
ways  of  connecting  the  cells  ?  310.  What  is  quantity?  Intensity? 
A  quantity  battery?  An  intensity  battery?  How  may  we  obtain 
both  quantity  and  intensity?  311.  Define  conductors  and  non- 
conductors. 312.  What  is  a  rheotome?  A  rheotrope?  Give  the 
origin  of  the  words.  313.  What  position  does  a  needle  take  with 
reference  to  a  conducting  wire?  314.  What  is  a  rheoscope? 
What  other  name  has  it?  On  what  principle  does  the  instru- 
ment depend?  315.  Describe  the  astatic  needle.  316.  What  law 
concerning  the  resistance  of  conductors?  317.  Show  that  the 
current  can  make  iron  magnetic.  What  is  a  helix?  An  electro- 
magnet? 318.  Show  that  the  wire  conducting  a  current  is  mag- 
netic. 319.  On  what  principles  do  electro-magnetic  engines 
depend?  Describe  Page's  rotating  apparatus.  320.  How  can 
electricity  be  made  to  regulate  the  motion  of  clocks?  321.  What 
is  a  telegraph?  What  four  things  essential  to  an  electric  tele- 
graph ?  What  is  the  receiving  instrument  in  the  needle  telegraph  ? 
In  Bain's  telegraph?  322.  Describe  the  receiving  instrument  of 
the  Morse  telegraph.  323.  What  is  said  of  the  earth  as  a  tele- 
graphic wire?  324.  What  is  the  purpose  of  the  relay?  De- 
scribe the  instrument.  325.  Describe  the  telegraphic  fire-alarm. 


324  APPENDIX. 

326.  Define  electrolysis,  electrolyte,  electrode,  anode,  and  cath- 
ode. What  compounds  may  become  electrolytes?  327.  De- 
scribe the  electrolysis  of  blue  vitriol.  328.  What  is  electrotyping? 
Describe  the  process.  Its  uses?  329.  What  is  electro-plating? 
Describe  the  process.  330.  Describe  electro-gilding.  331.  De- 
fine electro-metallurgy.  What  two  kinds?  Illustrate.  332. 
What  law  in  regard  to  the  heat  developed  by  a  current?  Ex- 
plain blasting  by  means  of  electricity.  333.  How  is  the  electric 
light  produced?  What  is  the  voltaic  arc?  What  is  said  of  its 
heat.  334.  Show  that  a  current  may  be  induced  by  a  magnet. 
335.  In  magneto-electric  machines,  how  is  the  electricity  in- 
duced? 336.  Describe  the  induction  coil  in  full.  337.  What  is 
the  extra  current?  338.  What  is  thermo-electricity?  How 
is  it  generated?  339.  Describe  the  thermopile.  Explain  its  use 
as  a  thermometer.  340.  What  is  frictional  electricity?  When  is 
it  developed?  341.  Describe  the  electrical  machine.  342.  What 
is  true  of  the  quantity  and  intensity  of  frictional  electricity? 
How  does  it  compare  with  voltaic  electricity?  343.  Describe 
the  electroscopes  mentioned.  Their  use?  344.  Show  that  the 
forces  on  the  two  conductors  act  in  opposite  directions.  345. 
What  are  these  forces  called,  and  what  is  true  of  their  develop- 
ment? 346.  What  is  induction?  Illustrate.  When  is  a  body 
polarized?  When  charged?  347.  Show  that  the  charge  on  a 
solid  conductor  is  on  the  surface.  348.  Describe  the  Leyden  jar. 
How  is  it  charged  ?  How  discharged  ?  349.  Describe  the  forms 
of  Leyden  battery.  How  do  they  differ  in  their  operation?  350. 
Explain  the  effect  of  points  on  a  conductor.  351.  Describe  the 
electric  wheel. 


INDEX. 


?or  concise  statements  of  the  leading  topics  of  the  book,  see  the  SUMMARIES, 
which  will  be  readily  found  by  means  of  the  Table  of  Contents.  The  references  in  this 
Index  are  only  to  the  fuller  treatment  of  subjects  in  the  body  of  the  work. 


Abb6  Nollet's  globe,  313. 
Action  and  reaction,  66. 
Adhesion,  6. 

of  gases  and  liquids,  24. 
solids,  23. 
liquids,  23. 

and  solids,  18,  20. 
solids,  1 8. 

Air  (see  Atmosphere). 
Air-gun,  the,  58. 
Air-pump,  the,  48. 
Annealing,  12. 
Anode,  214. 
Anthelia,  263. 
Apparatus,  list  of,  295. 
Artesian  wells,  41. 
Atmosphere,  buoyancy  of  the,  50. 

composition  of  the,  231. 

elasticity  of  the,  48. 

electricity  in  the,  256. 

heating  of  the,  231. 

moisture  of  the,  242. 

pressure  of  the,  52,  232. 

resists  the  fall  of  bodies,  63. 
Aurora  borealis,  the,  260. 


Balance,  the,  31. 

Balance-wheel,  the  compensation,  193. 

Balloons,  50. 

Barker's  mill,  90. 

Barometer,  the,  53. 

Battery,  Bunsen's,  199. 

Daniell's,  200. 

Grove's,  200. 

Leyden,  227. 

magnetic,  197. 

thermo-electric,  222. 
Beats,  115. 

Blue  vitriol,  electrolysis  of,  215. 
Boiling,  181. 
Brittleness,  8. 
Buoyancy,  42,  50. 


C. 

Calms,  region  of,  235,  252. 

Calorescence,  175. 

Camera  obscura,  the,  150. 

Capillarity,  21. 

Capstan,  the,  79. 

Cathode,  214. 

Centre  of  gravity,  the,  31. 

Chords,  1 1 6. 

Clocks,  construction  of,  71. 

electric,  208. 
Clouds,  246. 

colors  of,  264. 
Cohesion,  6. 

in  gases,  14. 
liquids,  12. 
solids,  6. 

Coils,  induction,  220. 
Collodion,  309. 
Color-blindness,  157. 
Colors,  142. 

complementary,  142. 

prismatic,  140. 
Condensation  of  vapors,  183. 
Condenser,  the,  58. 
Coronas,  262. 
Crank,  the,  93. 
Crystals,  10. 

Current,  the  electric,  199  (see  Electricity). 
Curves,  magnetic,  196. 


D. 

Daguerreotype,  the,  308. 
Density,  denned,  43. 
Dew,  243. 

Diamond  jar,  the,  314. 
Diathermancy,  173. 
Diffraction  fringes,  144. 
Discharge,  convective,  229. 

glow,  229. 

Discharger,  the,  228. 
Discords,  116. 
Ductility,  8. 
Dynamometer,  the,  7. 


326 


INDEX. 


Ear,  the  human,  129. 

range  of,  130. 

Earth,  as  a  telegraphic  wire,  211. 
Echoes,  103. 
Elasticity,  8. 
Electric  battery,  200. 
clocks,  208. 
lamp,  312. 
light,  218,  313. 
telegraph,  210. 
wheel,  229. 

Electrical  machine,  the,  224. 
Electricity,  a  source  of  mechanical  power, 

207. 

atmospheric,  256. 
conductors  of,  202. 
developed  by  friction,  223. 
heat,  222. 
magnetism,  219. 
develops  heat  and  light,  218. 
intensity  of,  201. 
makes  iron  magnetic,  206. 
nature  of,  274; 
positive  and  negative,  226. 
quantity  of,  201. 
resistance  to  current  of,  204. 
voltaic,  198. 
Electrodes,  214. 
Electro-gilding,  217. 
Electrolysis,  214. 
Electrolyte,  214. 
Electro-magnets,  206. 
Electro-metallurgy,  217. 
Electro-plating,  216. 
Electroscopes,  225. 
Electrotypmg,  215. 
Energy,  conversion  of,  275. 
kinds  of,  275. 
source  of,  280. 

transmuted,  not  destroyed,  280. 
Equilibrium,  32. 

of  moving  bodies,  61. 
Evaporation,  183,  242,  256. 
Expansion  by  heat,  184,  193,  311. 
Experiments,  notes  on,  295. 
Eye,  the,  151. 

adjustment  of,  152. 
affected  by  age,  162. 


F. 


Falling  bodies,  63. 
Farmer's  alloy,  222. 
Far-sightedness,  161. 
Fire-engine,  the,  56. 
Flames,  sensitive,  128. 
sounding,  127. 
Fluorescence,  175. 
Fly-wheel,  the,  95. 
Fogs,  244. 

Force-pump,  the,  54. 
Fraunhofer's  lines,  175,  273. 
Freezing-mixtures,  183. 
French  weights  and  measures,  285. 
Friction,  always  rhythmic,  127. 

electricity  developed  by,  223. 


G. 

Galvanometer,  the,  203. 
Gases,  buoyancy  of,  50. 

cohesion  in,  14. 

compressibility  of,  15. 

diffusion  of,  26. 

elasticity  of,  15. 

expansive  force  of,  48,  58. 

osmose  of,  26. 

pressure  of,  47. 

specific  gravity  of,  290. 
Gassiot's  cascade,  313. 
Geissler's  tubes,  313. 
Governor,  the,  93. 
Graham's  pendulum,  192. 
Gravity,  the  centre  of,  31. 

curves  the  path  of  projectiles,  62. 
increases  speed  of  falling  bodies, 

64. 

specific  (see  Specific  Gravity). 
Gulf  Stream,  the,  186. 
Gunpowder,  action  of,  58. 


H. 

Hail,  255. 

Halos,  264. 

Hardness,  8. 

Harmonics,  or  overtones,  1 1 1. 

Heat,  absorption  of,  175,  310. 

causes  liquids  to  boil,  180. 
solids  to  melt,  179. 

conduction  of,  177. 

convection  of,  185. 

conversion  of,  278. 

developed  by  electricity,  218. 

dispersion  of,  174. 

expansion  by,  184,  193,  311. 

latent,  180,  181. 

luminous,  173. 

mechanical  equivalent  of,  278. 

obscure,  173,  231,  311. 

of  the  atmosphere,  231. 

promotes  solution,  21. 

radiation  of,  173,  176, 310. 

reflection  of,  174. 

refraction  of,  174. 

solar,  282. 

specific,  179. 

the  same  as  light,  273. 

unit  of,  179. 
Heating  by  steam,  188. 
Helix,  206. 

Holtz's  electrical  machine,  301. 
Hot-houses,  188,  311. 
Hydrometer,  the,  44. 
Hydrostatic  press,  the,  38. 
Hygrodeik,  the,  194. 
Hygrometer,  the,  193. 

I. 

Iceland  spar,  144. 
Inclined  plane,  the,  85. 
Induction  coils,  220,  303. 
electrical,  227. 


INDEX. 


327 


Inductorium,  the,  303,  314. 

Inertia,  60. 

Instruments,  stringed,  117. 

wind,  119. 
Irradiation,  156. 


Lamp,  Bunsen's,  306. 

the  electric,  312. 
Lenses,  147. 

achromatic,  167. 
Level,  the,  59. 
Lever,  the,  74. 
Leyden  battery,  the,  227. 

jar,  the,  227. 
Light,  absorption  of,  142. 

chemical  action  of,  308. 
composition  of,  141. 
diffusion  of,  135. 
dispersion  of,  140. 
double  refraction  of,  144. 
intensity  of,  133. 
interference  of,  143. 
length  of  waves  of,  270. 
polarization  of,  145. 
radiation  of,  131. 
reflection  of,  134,  143. 
refraction  of,  134,  136,  273. 
total  reflection  of,  136. 
undulatory  theory  of,  269. 
velocity  of,  132. 
Lightning,  258. 

rods,  259. 

Liquids,  buoyancy  of,  42. 
cohesion  in,  12. 
C9mpressibility  of,  13. 
diffusion  of,  24. 
elasticity  of,  14. 
osmose  of,  24. 
pressure  of,  37. 
specific  gravity  of,  44. 
weight  of,  36. 


M. 

Machines,  74. 

law  of,  75. 

Magdeburg  hemispheres,  47. 
Magic  lantern,  the,  168. 
Magnetism,  195,  261. 

developed  by  electricity,  206. 
of  the  earth,  196. 
Magneto-electricity,  219. 
Magnets   195,  197. 
Malleability,  9. 
Mariotte's  law,  58. 
Mass,  defined,  65. 
Matter  acted  upon  by  gravity,  29. 

made  up  of  molecules,  3. 

states  of,  6. 

Melting-point,  the,  180. 
Microscopes,  165. 
Mirage,  136. 
Mirrors,  169 
Mists,  244. 
Molecular  forces,  5. 


Molecules  of  bodies  are  in  motion,  273. 

size  of,  3. 
Momentum,  65. 
Monsoons,  237. 
Motion,  60. 

laws  of,  60,  62,  65. 

quantity  of,  65. 

reciprocating,  92. 

reflected,  67. 

resistance  to,  61. 

rotary,  92. 
Musical  instruments,  117. 

sounds,  105,  112. 


N. 

Near-sightedness,  161. 
Nebular  hypothesis,  the,  283. 
Needle,  the  astatic,  203. 

the  dipping,  196. 
Nodal  lines,  no. 
Nodes  in  sounding  bodies,  109. 
Noise  defined,  105. 


O. 

Oceanic  currents,  185,  232,  244. 
Octave,  defined,  107. 
Opera-glass,  the,  167. 
Optic  nerve,  action  of  light  upon,  154. 
Optical  axis,  the,  157. 
Organ-pipes,  124. 
Osmose  of  gases,  26. 
liquids,  24. 
Overtones,  m. 

P. 

Page's  rotating  apparatus,  207. 
Parhelia  and  paraselenae,  264. 
Pendulum,  the,  69.  . 

Graham's,  192. 
Penumbra  of  shadow,  132. 
Photography,  308- 
Pisa,  the  Leaning  Tower  of,  34. 
Polarization  of  electricity,  227,  314. 

light,  145. 

Pressure  (see  Air,  Gases,  and  Liquids). 
Prisms,  134. 

achromatic,  140. 

double  refracting,  144. 

path  of  rays  through,  138. 
Problems,  287. 
Pulley,  the,  82. 
Pumps,  54. 

air,  48. 

R. 

Rack  and  pinion,  the,  78. 
Rain,  251. 
Rainbow,  the,  261. 
Ratchet,  the,  80. 
Rays  of  light,  132. 
Reaction,  66. 
Reed  pipes,  125. 
Relay,  the,  211. 


INDEX. 


Resonance,  120. 

Retina,  duration  of  impression  on,  155. 

structure  of,  153. 
Rheoscope,  the,  203. 
Rheotome,  the,  202,  312. 
Rheotrope,  the,  202,  312. 
Rods,  longitudinal  vibrations  of,  119. 
Kuhmkorit's  coil,  303,  314. 


Safety-valve,  the,  97. 

Saint  Elmo's  fire,  260. 

Screw,  the,  87. 

Shadows,  132. 

Simoom,  the,  241. 

Siphon,  the,  56. 

Siren,  the,  106. 

Snow,  253. 

Softness,  8. 

Solids,  adhesion  of,  18,  20,  23. 

cohesion  of,  6. 

compressibility  of,  10. 
Solution,  20,  21. 
Sonometer,  the,  108,  298. 
Sound,  caused  by  vibrations,  100. 

intensity  of,  102. 

interference  of,  114. 

quality  of,  112. 

reflection  of,  103,  267. 

refraction  of,  267. 

transmission  of,  101. 

velocity  of,  102,  103. 

will  not  pass  through  a  vacuum, 

100. 

Sounding-boards,  117. 
Sound-waves,  266. 
Speaking-tubes,  102. 
Specific  gravity,  43. 

of  a  gas,  290. 
Spectroscope,  the,  305. 
Spectrum,  the  solar,  130,  175. 
Spheroidal  state,  the,  182. 
Spirit  level,  the,  59. 
Spring  balance,  the,  30. 
Springs,  41. 
Steam  engine,  the,  91. 

locomotive,  97. 

heating  by,  188. 

latent  heat  of,  181. 
Steelyard,  the,  31. 
Stereoscope,  the,  160. 
Storms,  238. 
Strings,  vibrations  of,  108,  117. 

T. 

Tantalus's  cup,  57. 
Telegraph,  Bain's,  210. 

fire-alarm,  212. 

four  things  essential  to,  210. 

Morse's,  210. 

needle,  210. 


Telescope,  the,  166. 

reflecting,  171. 

refracting,  171. 

terrestrial,  167. 

Temperature  affects  time  pieces,  192. 
of  the  atmosphere,  232. 
Tempering,  12. 
Tenacity,  6. 

Thaumatrope,  the,  155. 
Thermo-electricity,  222. 
Thermometers,  190. 

differential,  191,  223. 
Thermopile,  the,  222. 
Thunder,  258. 
Thunder-storms,  257. 
Tornadoes,  239. 
Trade-winds,  235. 
Transparency,  131. 
Tubes,  vibrations  in,  122. 
Tuning-fork,  the,  106. 


V. 

Vacuum  defined,  50. 
Vibrations,  longitudinal,  119. 

of  columns  of  air,  122. 

sympathetic,  113. 
Vision,  laws  of,  160. 
Visual  angle,  the,  157. 
Voice,  the  human,  128. 
Voltaic  arc,  the,  218. 
electricity,  198. 
pair,  the,  198. 


W. 

Water,  electrolysis  of,  214. 
expansion  of,  187. 
frozen  by  evaporation  of  ether, 

3n- 

power,  89, 

specific  and  latent  heat  of,  187. 

wheels,  89. 
Waterspouts,  241. 
Wedge,  the,  86. 
Weight,  30. 

in  air,  50. 

liquids,  42. 
Wells,  Artesian,  41. 
Wheel  and  axle,  the,  79. 
Wheels,  kinds  of,  81. 
Wheel-work,  80. 
Whirlwinds,  239. 

dust,  240. 

Wind  instruments,  119. 
Windlass,  the,  79. 
Winds,  234. 
Wire,  9. 


Zoetrope,  the,  155. 


Cambridge :  Electrotyped  and  Printed  by  John  Wilson  and  Son. 


APPARATUS. 


The  following  articles  named  in  the  List  on  pages  295-303 
can  be  furnished  at  the  prices  affixed:  — 


COHESION. 

No.  i.   Bar  and  gauge     

.    .               $2.25 

„     2.   Two  half-pint  flasks  and  tubes  .    .    . 

.    .                 1.25 

„     3.   Lead  hemispheres    

.    .                 i.oo 

.      .     $1.00 

„     5.    Crucible      

.  .            .25 

.  .            .50 

„     7.    Two-quart  cylindrical  jar  

.  .            1.50 

„     8.    Dropping-tube     ........ 

.  .            .50 

„     9.   Six  test-tubes  

.  .            .50 

.  .    2.50 

„  ii.   Retort-and-  tube-holder     

.  .    3.50 

„  12.   Mercury  

.    .     5.00     5.00 

ADHESION. 

No.  i.   Glass  disk,  with  cord   

.  .           1.50 

.  .    1.50 

„     3.   Two  glass  funnels,  and  niters   .    .    . 

.      .                        I.OO 

„     4.    Set  of  capillary  tubes  

.      .                        I.OO 

„     5.   Pair  of  capillary  plates     

.  .    1.50 

.  .    1.50 

„     7.    Glass  funnel  tube     

.  .            .25 

.    .                1.75 

„     9.    Bottles  and  tube  

.      .                        I.OO 

„  10.    Cup  and  tube  

.      .                       I.OO 

APPARATUS. 


Fig.  i. 

MECHANICS. 

No.  I.  Illustrations  of  Centre  of  Gravity  (Fig.  i)  $5.00 

„     2.  Bottle  with  tubes     .    . $3.50 

3.  Liquid  equilibrium  tubes 3.50 

4.  Illustration  of  hydrostatic  press    ....  9.00 

5.  Hydrometer 1.25 

6.  Cylinder  and  cup 2.50 

7.  Ritchie's  school  air-pump  (Figure  2) 

The  plate  is  eight  inches  in  diameter,  and  forms  the  top  of 
a  cylinder  four  inches  high,  which  prevents  any  flexure 
or  change  in  its  form.  The  pump  cylinder  is  placed 
horizontally  beneath  the  plate,  and  is  thus  protected 
from  injury.  The  base  is  of  mahogany,  and  has  a  screw 
table  clamp.  This  pump  is  essentially  automatic  in  its 
action,  and  will  produce  a  vacuum  more  than  twice  as 
high  as  any  pump  with  valve  action ;  this  is  of  great 
importance  in  all  experiments  with  electricity.  It  is 
worked  with  ease,  and  is  warranted  to  be  trustworthy 
and  durable.  A  patent  has  been  applied  for.  25.00 

„    8.   Sliding-rod  receiver,  complete 5.00 

„    9.  Plain  quart  receiver i.oo 

„  10.  Apparatus  for  weight  and  buoyancy  of  air 

(See  Figure  2) 7.50 

„  ii.   Magdeburg  hemispheres 7.50 

„  12.   Hand  glass 1.25 

„  13.  Rubber  bag,  with  screw  cap  and  hook   .    .  2.25 


APPARATUS. 


Fig.  2. 

No.  14.   Barometer  tube $1.50 

„     15.    Model  of  lifting  pump 9.00 

„  1 6.  Model  of  force  pump,  with  stand  and  cis- 
tern for  both  pumps $13.50 

„     17.    Siphon 0.50 

„     1 8.    Tantalus's  cup 2.00 

„     19.    Condenser 9.00 

„     20.    Condensing    chamber,  with   air-gun  and 

jets  for  water 9.50 

„     21.    Guinea  and  feather  tube  (fitted  also  for 

electrical  experiments) 8.00 

„     22.    Illustration  of  pendulum 3.50 

„     23.   Illustration  of  central  forces 3.75 

„     24.   Models  of  simple  and  compound  levers    .  5.50 

„  25.  Models  of  wheel  and  axle,  capstan,  and 
pulleys,  with  stand  and  set  of  weights 
(Figure  3). 

This  arrangement  for  the  illustration  of  these  simple  ma- 
chines will  be  found  very  convenient  and  complete.  The 
teacher  can  thus  have  before  his  class  those  only  that 
illustrate  the  lesson.  2O.OO 

„     26.    Models   of   screw,   wedge,   and   inclined 

plane,  with  car 7.50 


APPARATUS. 


Fig-  3- 

No.  27.   Model  of  Barker's  mill $2.25 

„     28.   Wollaston's   illustration  of  low-pressure 
engine. 

This  is  a  cylinder  with  a  thin  copper  globe  attached,  piston 
and  rod,  and  handle.  Pour  a  teaspoonful  of  water  into 
the  globe,  and  hold  over  a  spirit-lamp ;  steam  will  be 
generated  and  drive  up  the  piston  :  then  plunge  the  globe 
in  cold  water  ;  the  steam  will  be  condensed  and  the  pis- 
ton forced  down  by  atmospheric  pressure.  5'5^ 


SOUND. 

No.  i.  Bell  for  vacuum 

„     2.  Revolving  toothed  wheel .     .     . 

„     3.  Tuning-fork  and  case  .... 

„     4.  Sonometer,  with  wires,  complete 

„     5.  Violoncello  bow 

„     6.  Vibrating  plate 

„     7-  Four  rods  on  bar 

„     8.  Brass  rods  . 


3-50 


$3-25 

7.50 

n.oo 

25.00 

2.00 
3-50 

1.50 


APPARATUS. 


Fig.  4. 


Fig-  6. 


Fig.  5- 


Fig.  8. 


Fig.  7- 


No.  9.  Iron  screw-press  for  Nos.  6,  7,  and  8  (Fig- 
ure 4) $7-50 

„  10.  Ivory  ball  and  stand,  for  No.  8      ....                 2.25 

„  ii.    Resonant  jar 2.00 

„  12.   Three  glass  tubes $0.50 

„  13.  Organ  pipe,  with  sliding  piston      ....                 4.50 

„  14.  Reed  pipe,  with  glass  chamber      ....     4.75 

„  15.  Jet  for  singing  flame i.oo 

LIGHT. 

No.  i.   Condensing  lens,  mounted 10.00 

„     2.   Prism,  for  refraction 2.00 

„     3.   Prism,  No.  2,  mounted  (Figure  5),  $6.00 ; 

(difference  compared  with  No.  2)   ...     4.00 

„     4.   Achromatic  prism 7.25 

„     5.   Revolving  disk  apparatus,  with  disks    .  - .  IO-75 

„    6.   Apparatus  for  Newton's  rings,  brass  frame  6.50 

„     7.   Zoetrope 5.00 

„     8.   Stereoscope,  with  12  diagrams 4.50 

„    9.   Convex  and  concave  mirrors,  ground  and 

polished 4.50 

„  10.   Mounted  mirror 4.25 

„  ii.   Double-convex  and  double-concave  lenses  3.00 


APPARATUS, 


Fig.  12. 


Fig.  9. 


Fig.  10. 


Fig.  xi. 


HEAT. 

No.  i.    Iodine  cell $5.00 

„     2.    Differential  thermometer $3.50 

„     3.    Conductometer 4.50 

„     4.  Pair  of  plates,  with  copper  and  tin  balls     .  3.25 

„     5.    Compound  bar 1.25 

„     6.  Illustration  of  convection  of  gases    .     .     .  3.50 

„     7.    Mason's  hygrometer 4.00 

„     8.    Edson's  hygrodeik 15.00 

„     9.   Pair  of  reflectors,  and  ball 12.50 

„  10.    Fire  syringe  and  tinder 3.00 

„  ii.   Spirit-lamp i.oo 

ELECTRICITY. 

No.  i.    Bar  magnet  (Figure  6) i.oo 

„     2.  U-magnet  (Figure  7)    ........  i.oo 

„     3.   Voltaic  pair 1.50 

„     4.   Bunsen's  cell .  4.50 

„     5.   Magnetic  needle  (Figure  8) 1.50 

„    6.   Dipping  needle 2.50 


APPARATUS. 


Fig.  13- 


Fig.  14. 


Fig.  15. 


No.  7.   Oersted's  galvanometer  (Figure  9). 

A  base  and  pillar  supporting  a  wire  frame,  within  which  is 
suspended  a  magnetic  needle  on  a  centre  point.  The 
wire  has  three  pole  cups  arranged  so  that  the  battery 
current  may  pass  above,  below,  or  around  the  needle. 
The  frame  must  be  placed  north  and  south.  $4-75 

„  8.  Electro-magnet 3.00 

„  9.  Lifting  coil 3.00 

„  10.  Page's  rotating  apparatus 8.00 

„  ii.  Helix  and  ring  (Figure  10). 

Two  semi-circular  pieces  of  soft  iron,  with  handles,  and 
a  helix  of  copper  wire.  Connect  the  helix  with  the  bat- 
tery, and  great  force  is  required  to  separate  the  ring.  $4-25 

„  12.   Model  of  telegraph  (Figure  1 1)      ....  8.00 

„  13.    Model  of  relay  magnet 9.50 

„  14.    Decomposing  cell 3.25 

„  15.   Thermo-electric  series 2.00 

„  1 6.  Vibrating  shocker,  or  induction  coil  (Fig- 
ure 12). 

A  helix  enclosing  an  iron  rod,  or  bundle  of  wires,  with  a 
vibrating  break-piece  ;  another  helix,  of  great  length, 
of  fine  wire,  entirely  separated,  surrounds  the  former.  7-5° 

„  17.  Separable  helices,  $  1 8.00;  (difference,  com- 
pared with  No.  1 6) 10.50 

„  1 8.  Powder  cup  (for  explosion  of  powder  by 

battery) i-oo 

„  19.   Vulcanite  cylinder  (for  friction)      ....  1.25 

„  20.   Electrical  machine        25.00 


8  APPARATUS. 

No.  21.  Holtz's  machine,  $50.00  ;  (difference  com- 

pared with  No.  20)     .......  $25.00 

„     22.  Insulated  conductor  ........    12.50 

„    23.  Gold-leaf  electroscope    .    ......     6.00 

„     24.  Leyden  jar    ............  $2.00 

„     25.  Diamond  jar  (Figure  13)     ......      3.50 

„     26.  Discharger    ...........  2.00 

„     27.  Electric  wheel   .......    ...  1.25 

„     28.  Spotted  tube  (for  electric  light)    ....     4.00 

„    29.  Stand  and  bells  (Figure  14). 

A  basement  with  pillar  and  bell,  and  a  similar  bell  to  screw 
to  the  stem  of  a  Leyden  jar.  A  little  ball  suspended 
between  the  bells  will  be  alternately  attracted  and  re- 
pelled between  them,  vibrating  for  a  long  time,  and 
gradually  discharging  the  jar.  5'^Q 

„    30.   Gassiot's  cascade  (Figure  15)  .....     2.50 
„    31.   Geissler's  tubes,  $2.00  to    .    .....     5.00 


SUMMARY. 

COHESION    .................  $12.75 

ADHESION   ................  7  50 

MECHANICS     ...............  124.50 

SOUND     .....    .    .    .........    .  70.00 

LIGHT  ..........    ........  26.75 

HEAT  ..................  24.00 

ELECTRICITY    ...............  84.50 

TOTAL  (for  articles  priced  in  right-hand  column)  .  $350.00 


A  charge  for  boxing  and  packing  will  be  made  of  two  per  cent 
on  the  amount  of  the  bill. 

The  above  list  of  apparatus,  or  any  considerable  portion  of  it, 
will  be  delivered  free  of  freight,  and  insured  against  dangers  of 
transportation  or  breakage,  if  a  draft  on  Boston  or  New  York  is 
received  with  the  order. 

Orders  maybe  sent  to  E.  S.  RITCHIE  &  SONS,  Manufacturers, 
149  Tremont  Street,  Boston ;  or  to  WOOLWORTH,  AINSWORTH,  & 
Co.,  New  York. 


NEW    AND    VALUABLE 

TEXT-BOOKS   IN  PHYSICS 


FOR 


GRAMMAR  AND  DISTRICT  SCHOOLS,  HIGH 
SCHOOLS,    AND  ACADEMIES, 


BY 


W.  J.  ROLFE  AND  J.  A.  GILLET, 

TEACHERS    IN    THE     HIGH    SCHOOL,    CAMBRIDGE,    MASS. 


THE   CAMBRIDGE   COURSE   IN   PHYSICS 

IN   THREE   VOLUMES : 

I.    CHEMISTRY,  $2.00. 
II.    NATURAL    PHILOSOPHY,  $2.00. 
III.    ASTRONOMY,  $2.00. 

$iT  New  and  revised  editions  of  these  books  have  been 
prepared,  and  the  Series  is  now  complete  in  a  permanent  form* 

The  Electricity  of  the  old  "  Chemistry  and  Electricity  "  has 
been  transferred  to  the  '"  Natural  Philosophy  "  in  the  new  edi- 
tion, and  has  been  wholly  rewritten,  made  somewhat  briefer, 
and  brought  fully  down  to  the  present  state  of  the  science. 
There  has  also  been  added  to  the  Appendix  of  the  "  Natural 
Philosophy  "  a  chapter  on  the  Physics  of  the  Atmosphere,  or 
Meteorology,  containing  all  the  recent  discoveries  and  theories 
in  this  important  and  interesting  field. 

As  thus  revised,  the  "  NATURAL  PHILOSOPHY  "  is  complete 


in  itself,  containing  Mechanics  (under  which  head  are  included 
Hydrostatics,  Hydraulics,  Pneumatics,  Motion,  Machines,  etc.), 
Sound,  Light,  Heat,  Electricity,  and  Meteorology. 

The  "CHEMISTRY"  has  been  carefully  rewritten  and  ex- 
panded so  as  to  fill  the  space  occupied  by  the  Electricity  in  the 
old  edition.  New  chapters  on  Crystallography  and  Organic 
Chemistry,  from  the  freshest  sources,  have  been  added,  and  the 
description  of  Elements  has  been  enlarged. 

This  edition  (June,  1869)  gives  the  nomenclature  as  adopted 
by  the  London  Chemical  Society,  as  taught  at  Harvard  College, 
and  as  generally  used  in  scientific  journals. 

The  new  edition  of  the  "ASTRONOMY  "  contains,  in  addition 
to  the  Astronomy  proper,  a  chapter  on  the  Conservation  of 
Energy  and  an  account  of  the  Constellations,  illustrated  by  17 
full-pa^e  Star  Maps  from  Argelander. 

These  books  are  inductive  in  method,  fresh  in  matter,  simple 
in  style,  fully  illustrated,  and  handsomely  printed,  and  they  ex- 
actly meet  the  wants  of  our  advanced  Seminaries  and  Acade- 
mies, and  of  those  High  Schools  which  can  devote  considera- 
ble time  to  these  subjects. 


THE    HANDBOOK    SERIES 

IN   THREE  VOLUMES : 

I.  HANDBOOK  OF  CHEMISTRY,  $  1.25. 
II.  HANDBOOK  OF  NATURAL  PHILOSOPHY,  $  1.25. 
III.  HANDBOOK  OF  THE  STARS,  $1.50. 

These  books  contain  (aside  from  the  Appendix)  respectively 
159,  230,  and  159  pages,  in  clear,  open  type,  with  no  fine  print, 
and  they  treat  of  all  the  topics  usually  included  in  school 


manuals  of  these  sciences.  The  more  theoretical  portions  of 
the  subject  are  discussed  briefly  in  Appendixes,  and  descrip- 
tions of  apparatus  and  directions  for  performing  experiments 
are  added.  Omitting  the  Appendixes,  the  books  are  not  too 
difficult  for  the  upper  classes  in  Granxmar  and  District  Schools. 
With  the  Appendixes,  they  are  exactly  adapted  to  the  wants  of 
those  High  Schools  and  Academies  which  have  not  time  for 
larger  books. 

They  are  not  abridgments  of  the  larger  works  by  the  same 
authors,  but  are  wholly  new  and  independent  books,  differing 
from  the  others  in  the  selection,  arrangement,  and  treatment  of 
topics,  so  far  as  w^is  necessary  to  fit  them  for  a  briefer  and 
easier  course  of  study.  They  are  simple  in  style,  and  emi- 
nently ^rad&a/,  yet  thoroughly  scientific,  and  giving  the  results 
of  the  latest  discovery  and  research.  They  are  sure  of  a 
hearty  welcome  from  teachers  who  desire  books  that  shall  be 
brief  without  being  dry,  and  easy  without  being  puerile. 

IgOr"  E.  S.  RITCHIE  &  SONS,  of  Boston,  will  furnish  a  set  of 
apparatus  for  the  thorough  illustration  of  the  Handbook  of 
Chemistry  for  $30,  and  a  set  for  the  thorough  lustration  of  the 
Handbook  of  Natural  Philosophy  for  $  350. 


***  Circulars,  containing  notices  and  testimonials  from  emi- 
nent teachers,  will  be  furnished  on  application.  Copies  for 
examination  will  be  supplied  at  one  half  the  advertised  price, 
with  twenty-five  cents  additional  for  postage.  Special  terms 
will  be  given  for  first  introduction  of  any  of  the  books. 

WOOLWORTH,  AINSWORTH,  &  CO., 

NEW  YORK. 


TESTIMONIALS. 


BELOW  we  give  extracts  from  a  few  of  the  most  recent  testimonials 
to  the  merits  of  this  popular  series  :  — 

The  Pennsylvania  School  Journal  (April,  1869,)  speaks  thus  of  the 
series  :  "  The  progress  of  science  teaching  in  our  schools  has  un- 
doubtedly been  retarded  by  the  lack  of  suitable  text-books.  That 
want  has  been,  in  a  great  measure,  velieved  by  the  recent  labors  of 
Messrs.  Rolfe  and  Gillet,  in  their  double  course,  —  a  course  of  Hand- 
books for  those  who  do  not  desjre  anything  beyond  elementary  in- 
struction, and  another  of  more  comprehensive  text-books  on  the  same 
sciences,  —  the  last  of  which  has  just  been  published.  These  text- 
books commend  themselves  as  the  work  of  men  whose  experience  in 
the  class-room  has  taught  them  the  most  effective  methods  of  present- 
ing scientific  truth,  and  whose  design  has  been  to  present  the  results 
of  the  latest  investigations  in  the  several  departments  of  each  science 
treated.  They  aim  throughout  at  nothing  more  than  clearness,  nothing 
less  than  accuracy.  We  remark,  especially,  the  absence  of  any  loose 
statement  which  could  mislead  the  pupil  or  leave  a  half-formed  idea. 
Teachers  all  know  the  difficulty  of  banishing  a  false  impression  which 
has  for  a  time  been  accepted  and  applied  as  fact.  Instead  of  the 
weakness  which  tends  to  enfeeble  the  growing  mind  by  presenting  the 
study  of  science  as  a  play  lesson,  —  a  mere  succession  of  interesting 
experiments,  etc.,  —  we  have  scientific  truth  here  set  forth  as  a  study 
which  the  schoolboy  may  feel  an  honest  pride  in  mastering.  In  none 
of  these  works  is  system  sacrificed  to  simplicity,  as  in  some  others  of 
their  class ;  yet  the  text-books  of  the  Cambridge  Course  yield  to  none 

in  point  of  interest At  the  end  of  each  division  of  the  subject, 

a  carefully  prepared  Summary  has  been  inserted,  thus  binding  together 

and  again  classifying  all  the  matter  previously  given Indeed, 

take  the  Cambridge  Physics  throughout,  the  Course  is  greatly  in  ad- 
vance of  any  heretofore  issued  in  this  country.     The  publishers  have 


TESTIMONIALS. 


also  done  their  part  well.  It  is  a  luxury  to  sweep  the  hand  over  these 
smooth,  solid,  glossy  pages,  —  beautiful  also  to  the  eye,  in  their  old- 
style  type  and  in  their  wealth  of  scientific  illustration,  —  and  think  of 
the  books  as  specially  designed  for  our  Common  Schools." 


The  San  Francisco  Bulletin,  (March  20,  1869,)  in  a  review  of  the 
whole  series,  says :  "  These  works  embody  the  latest  results  of  sci- 
entific discovery.  The  compilers  wisely  discard  theory,  however 
plausible  and  fascinating,  unless  it  rests  on  a  solid  groundwork  of 
truth.  The  arrangement  and  divisions  of  subjects  are  judicious  ;  the 
method  of  teaching  is  based  on  correct  principles.  The  definitions 
are  clear  and  concise  ;  the  notes  are  models  of  perspicuity.  There  is 
no  useless  verbiage ;  no  vague  generalization  ;  no  rhapsodies  of  style. 
The  student  is  directly  introduced  to  the  subject,  and  kept  rigidly  to 
it  until  he  has  mastered  it.  At  the  close  of  each  section  the  ground 
gone  over  is  reviewed,  and  the  principles  educed  are  summed  up  in  a 
few  pithy  axioms.  While  these  volumes  are  more  immediately  de- 
signed for  the  use  of  schools,  they  have  high  claims  to  recognition  as 
scientific  treatises.  We  can  hardly  think  of  any  other  souree  where 
so  much  valuable  knowledge  of  physics  can  be  found  in  so  short  a 
compass.  The  subjects  are  discussed  with  a  freshness  and  oftentimes 
a  grasp  of  thought  rare  in  books  of  the  kind." 


The  Boston  Courier  says  of  the  whole  series  :  "  They  are  in  ad- 
vance, by  a  great  stride,  of  other  text-books  in  common  use,  not  only 
because  they  pursue  the  only  rational  method,  so  long  ago  instituted 
by  Bacon,  and  yet  so  much  neglected,  of  deducing  principles  from 
facts,  instead  of  supporting  rules  laid  down  by  examples,  but  also  be- 
cause they  have  kept  pace  with  the  progress  of  constantly  accumulat- 
ing knowledge  and  furnish  us  with  the  latest  results.  They  are  MODEL 
BOOKS." 


The  Boston  Daily  Advertiser  says  :  It  is  in  their  attempt  to  keep 
up  with  the  progress  and  present  condition  of  scientific  knowledge, 
that  these  volumes  differ  most  widely  from  the  text-books  generally 
used  in  our  schools.  Chemistry  and  electricity  —  heat,  light,  and 
sound  —  wear,  in  this  Cambridge  Course  of  Physics,  a  very  different 
aspect  from  that  which  they  present  in  similar  treatises  of  five-and- 


TESTIMONIALS. 


twenty  years  ago.  ....  We  rejoice  to  believe  that  many  myriads  of 
the  young  —  and  not  of  the  young  only  —  will  here  be  introduced  to 
discoveries  and  speculations  of  so  much  interest  and  grandeur." 


The  Commonwealth  (Boston),  in  noticing  the  "  Handbook  of  Natu- 
ral Philosophy,"  says  :  "  The  merit  of  this,  as  of  the  other  volumes, 
is  that  it  breaks  away  from  the  traditional  method  of  preparation  of 
such  works,  and  by  original  arrangement,  the  introduction  of  the  latest 
discoveries  and  experiments,  and  the  amplest  illustration,  gives  at  once 
the  most  complete  and  accurate  data  of  the  subject-matter  treated.  It 
is  fresh  and  pertinent  throughout,  and  is  equally  valuable  in  school, 
counting-house,  or  family." 


The  Boston  Journal  of  Chemistry  (March,  1869,)  says  of  the  series  : 
"  We  have  carefully  examined  these  books,  and  find  them  to  be 
compiled  with  great  accuracy  and  care  ;  and  the  arrangement  of  top- 
ics and  the  general  style  are  admirable.  The  pleasing  perspicuity  and 
commendable  exactness,  with  which  the  elementary  principles  and 
facts  of  the  physical  sciences  are  presented  in  these  treatises,  are  in 
marked  contrast  with  a  number  of  text-books  which  have  somehow 
found  their  way  into  many  of  our  schools." 


The  Round  Table  (New  York)  says  of  the  "  Handbook  of  the 
Stars " :  "  It  is  a  very  admirable  specimen  of  the  abilities  of  the 
authors.  There  is  scarcely  anything  in  it  which  young  pupils  cannot 
readily  comprehend.  The  illustrations  are  really  beautiful,  and  the 
collection  of  celestial  maps  at  the  end  adds  greatly  to  the  value  of  the 
work.  An  Appendix  discusses  with  considerable  thoroughness  some 
of  the  more  abstruse  subjects  touched  upon  in  the  body  of  the  book. 
The  name  scarcely  expresses  the  full  scope  of  this  manual.  It  is 
really  an  elementary  treatise  on  astronomy  without  mathematics,  and 
is  very  good  reading  for  any  one  with  a  taste  for  science  but  neither 
the  time  nor  the  inclination  to  go  de'eply  into  its  study.  It  is  brought 
down  to  the  very  latest  dates,  —  another  advantage  in  these  days  of 
stereotype  plates  and  non-revision.  We  hope  that  Messrs.  Rolfe  and 
Gillet  will,  at  least  once  in  every  three  or  four  years,  go  carefully  over 
their  text-books  and  bring  them  down  to  the  time  of  the  latest  cor- 
rections and  discoveries.  By  so  doing,  they  will  very  effectually  keep 
the  start  of  those  lazy  writers  who,  when  they  have  finished  a  school- 


TESTIMONIALS.  7 

book  on  a  progressive  science,  imagine  it  is  to  last  without  revision  as 
long  as  they  remain  in  the  world  to  draw  an  income  from  its  sales." 


Sillimaifs  Journal  (March,  1869,)  says  of  the  "Handbook  of  Chem- 
istry "  :  "  On  the  whole,  the  book  is  a  valuable  addition  to  our  mea- 
gre collection  of  text-books  on  the  new  system,  and  we  commend  it  to 
the  notice  of  teachers." 

La  Renaissance  Louisianaise  (New  Orleans)  says  of  the  "  Natural 
Philosophy  "  :  "  Get  ouvrage  instructif  convient  a  1'usage  des  gens 
du  monde  aussi  bien  qu!a  celui  des  etudiants.  C'est  un  manuel  qui 
traite  avec  une  grande  clarte  demonstrative  sur  tous  les  sujets  ele- 

mentaires  de  la  Physique,  d'apres  les  plus  recentes  decouvertes 

Les  auteurs  ont,  selon  nous,  completement  reussi  a  produire  un  ou- 
vrage utile  au  plus  haut  point  et  dont  personne  ne  devrait  se  dispenser." 


Mr.  S.  M.  Capron,  of  the  High  School,  Hartford,  Conn.,  where  the 
"  Natural  Philosophy "  has  been  adopted,  says  :  "  We  have  ex- 
amined all  the  recent" text-books  on  this  subject  which  have  appeared, 
and  feel  convinced  that  this  is  the  best  arranged  of  all  for  our  purpose, 
and  most  fully  up  to  the  present  state  of  scientific  research." 


Professor  Edward  Conant,  of  the  Vermont  State  Normal  School, 
writes  that  his  pupils  have  used  the  same  book  "  with  constant  de- 
light, and,  of  course,  with  profit." 


Mr.  L.  R.  Williston,  of  Cambridge,  Mass.,  writes  thus  :  "  I  will 
express  my  good  opinion  of  the  '  Handbook  of  Natural  Philosophy ' 
by  simply  saying  that  I  intend  to  use  it  in  my  school.  I  shall  also 
continue  to  use  the  '  Handbook  of  the  Stars,'  and  shall  use  your  book 
in  Chemistry,  if  I  use  any." 


Mr.  W.  B.  Stickney,  Master  of  the  High  School,  Chicopee,  Mass., 
says  :  "  The  '  Handbook  of  the  Stars '  bears  the  test  of  the  school- 
room. My  class  is  delighted  with  it." 


8  TESTIMONIALS. 

Professor  W.  S.  Smyth,  of  Wyoming  Seminary,  Kingston,  Pa.,  who 
has  adopted  the  same  book,  says  :  "  For  logical  arrangement,  clear- 
ness of  expression  and  illustration,  as  well  as  for  mechanical  execution, 
it  is  unsurpassed." 


Professor  John  B.  Bttrwell  of  the  Charlotte  Female  Institute,  North 
Carolina,  writes  :  "  I  have  been  using  the  books  for  the  last  two  or 
three  years,  and  consider  them  superior  in  all  respects  to  any  others  I 
have  ever  met  with.  I  can  also  add  the  recommendation  of  Dr. 
Philips,  whose  reputation  as  a  teacher  is  known  throughout  the 
South." 


THE  following  is  from  the  official  report  of  the  regular  meeting  of 
the  Chicago  Board  of  Education,  May  7,  1869  :  — 

"  Mr.  Carter  moved  to  adopt  the  next  recommendation  of  the  Com- 
mittee, to  wit :  Rolfe  and  Gillet's  Chemistry,  in  place  of  Wells's  Chem- 
istry. 

"  Carried.  Yeas  —  Messrs.  Ballahtyne,  Bond,  Bonfield,  Briggs, 
Carter,  Guilford,  Holden,  King,  Meserve,  Runyan,  Shackford,  Tink- 
ham,  and  Walsh  —  13.  Nays  —  None." 


***  This  popular  course  of  Physics  has  been  officially  adopt- 
ed by  the  State  Board  of  Maryland  and  Minnesota,  and  is  al- 
ready used,  in  whole  or  in  part,  in  the  cities  of  Baltimore,  Pitts- 
burg,  Wheeling,  Richmond,  Savannah,  Charleston,  Mobile, 
New  Orleans,  Galveston,  Memphis,  Nashville,  Louisville,  St. 
Louis,  Chicago,  Milwaukee,  Racine,  Bloomington,  Detroit,  Cin- 
cinnati, Columbus,  Dayton,  Cleveland,  St.  Joseph,  Wheeling, 
Buffalo,  Rochester,  Newark,  Worcester,  Taunton,  Lowell,  Ban- 
gor,  Lawrence,  Haverhill,  Bath,  Milford,  Hartford,  New  London, 
New  Bedford,  Boston,  Cambridge,  Dover,  Concord,  Nashua, 
Burlington,  Dorchester,  Manchester,  Pittsfield,  Chelsea,  Chico- 
pee,  Northampton,  San  Francisco,  etc.,  etc. 


THE  mw  LATIN  COURSE. 


PREPARATORY  LATIN  PROSE  BOOK, 

CONTAINING 

ALL  THE   LATIN    PROSE    NECESSARY   FOR    ENTERING   COLLEGE;    WITH 

REFERENCES   TO   THE   GRAMMARS   OF  HARKNESS,  ANDREWS 

AND  STODDARD,    ALLEN,   AND   BULLIONS. 

BY  J.  H.  HANSON,  A.  M., 

PRINCIPAL     OF     THE     WATERVILLE     CLASSICAL     INSTITUTE. 


II. 

A  HANDBOOK  OF  LATIN  POETRY, 

CONTAINING 

SELECTIONS    FROM   VIRGIL,    OVID,    AND   HORACE;    WITH   NOTES, 

AND.  REFERENCES   TO   THE   GRAMMARS   OF   HARKNESS, 

ANDREWS    AND     STODDARD,    ALLEN, 

AND   BULLIONS. 

BY  J.  H.  HANSON,  A.  M.,  AND  W.  J.  ROLFE,  A.  M. 


III. 

SELECTIONS  FROM  OVID  AND  VIRGIL. 

A   SHORTER    HANDBOOK   OF   LATIN    POETRY  ;    WITH   NOTES 
AND   GRAMMATICAL   REFERENCES. 

BY  J.  H.  HANSON,  A.  M.,  AND  W.  J.  ROLFE,  A.  M. 

This  volume  comprises  all  the  Latin  Poetry,  Notes,  and  References  contained  in  the 
larger  volume,  with  the  exception  of  Horace. 


No.  1  embraces  all  the  Latin  prose  requisite  to  preparation  for  col- 
lege, with  reference  to  the  grammars  most  in  use,  Critical  and  Explan- 
atory Notes,  and  a  Vocabulary. 

No.  2  completes  the  plan,  —  furnishing  all  the  requisite  Latin 
poetry.  The  two  volumes  comprise  all  the  Latin  necessary  to  be  read 
in  preparing  for  a  collegiate  course,  and  all  that  is  needed  to  complete 
the  Latin  reading  of  pupils  who  terminate  their  classical  studies  in  our 
High  Schools  and  Academies. 

No.  3  is  the  same  as  No.  2,  omitting  the  Selections  from  Horace. 


MAGILL'S  FRENCH  COURSE. 


I. 

A  FRENCH  GRAMMAR: 

BEING  AN   ATTEMPT    TO    PRESENT,    IN    A   CONCISE    AND     SYSTEMATIC 
FORM,   THE   ESSENTIAL   PRINCIPLES   OF  THE   FRENCH   LAN- 
GUAGE :    TO    WHICH     IS    ADDED    A    FRENCH, 
ENGLISH,  AND  LATIN  VOCABULARY. 

Eleventh  Edition.     Enlarged  and  Improved. 


II. 

AN  INTRODUCTORY  FRENCH  READER 

CONTAINING 

SELECTIONS    FOR    READING    AND    DECLAMATION. 


III. 

FRENCH  PROSE  AND  POETRY: 

BEING  AN  ADVANCED  FRENCH  READER  : 

CONTAINING 

SELECTIONS    FROM    THE     PRINCIPAL  CLASSICAL    FRENCH     POETS   AND 
PROSE-WRITERS    DURING   THE   PAST  TWO   HUNDRED  YEARS, 
WITH    BIOGRAPHICAL    NOTICES    OF    THE    AUTHORS; 
THE  WHOLE  CHRONOLOGICALLY  ARRANGED. 

BY  EDWARD  H.  MA  GILL,  A.  M., 

PROFESSOR   OF   ANCIENT   AND   MODERN   LANGUAGES    IN   SWARTHMORE   COLLEGE, 
PENNSYLVANIA. 


No.  1  now  contains  a  complete  course  of  Grammar,  illustrated  by  copious  exer- 
cises, English-French  and  French-English,  together  with  a  very  full  treatise  on 
pronunciation,  brought  down  to  date  according  to  the  most  recent  authorities. 

No.  2  Contains  selections  progressively  arranged  ;  and  its  very  full  vocabulary 
gives  the  derivations  of  the  words  as  well  as  their  definitions  and  pronunciation,  an 
entirely  new  feature  in  a  work  of  this  character.  Both  this  work  and  No.  i  are 
highly  recommended  by  M.  Bescherelle  aine",  author  of  the  Dictionnaire  National. 

No.  3  is  a  combination  of  the  best  materials  to  make  a  useful  French  Reader  which 
the  author  could  obtain  during  a  residence  of  some  months  in  France,  some  of  them 
already  widely  used  in  the  French  schools,  and  others,  new  selections,  taken  from 
the  original  sources. 

The  whole  series  forms  a  very  complete  course  of  instruction  in  French,  according 
to  the  most  approved  modern  method,  for  our  schools  and  colleges. 

10 


A 

NEW  ELEMENTARY  COURSE 


IN 

THE   GERMAN  LANGUAGE, 

FOR  THE  USE  OF  SCHOOLS. 

BY 

GABRIEL   CAMPBELL,  M.A., 

PROFESSOR    IN    THE    STATE    UNIVERSITY    OF    MINNESOTA. 
I2tno.     pp.  200. 


THE  aim  of  this  work  is  to  make  a  practical  application  of 
the  improvements  developed  by  the  growth  of  the  modern  science 
of  Comparative  Philology. 

The  author  presents  the  German  language  to  American  learners, 
who  are  presumed  to  be  acquainted  with  the  English  language,  by 
way  of  comparison  with  the  English  in  its  points  of  similarity  and 
of  difference. 

The  plan  is  simple,  philosophical,  and  practical,  and  the  work  is 
proving  itself  eminently  successful.  It  has  received  very  flattering 
encomiums  from  high  authorities  in  all  parts  of  the  country  where 
German  is  taught. 

The  book  is  divided  into  three  parts :  — 

PART  I.    General  Principles  ; 

PART  II.  Synopses  —  Forms  of  Words  ; 

PART  III.   Special  Principles,  Reading  and  Analysis ; 

followed  by  a  German  and  English  Vocabulary  to  Part  III.     It  con- 
tains also  an  English  and  German  Vocabulary  to  Part  I, 

11 


BARTHOLOMEW'S 

DRAWING    SERIES, 

DESIGNED   FOR  THE 

PKIMAEY,  GRAMMAR,  AND  HIGH  SCHOOL. 

BY  WILLIAM  N.  BARTHOLOMEW, 

PROFESSOR   OF   DRAWING    IN    THE   ENGLISH    HIGH    AND   GIRLS*    HIGH   AND 

NORMAL   SCHOOLS,    AND    DIRECTOR   OF   DRAWING   IN    THE 

GRAMMAR    SCHOOLS    OF    BOSTON. 


Bartholomew's  Primary-School  Writing  and  Drawing 

Slate. 
Bartholomew's  Drawing-Books.    NEW  SERIES.    In  twelve 

numbers;  with  "Teacher's  Guide"  for  No.  I,  No.  2,  No.  3,  and 
No.  4. 

Bartholomew's  Drawing-Cards  for  Blackboard  Use. 

Bartholomew's    Progressive    Picturesque    Drawing- 
Cards.     In  four  numbers. 

Bartholomew's  Linear  Perspective.    One  vol.  8vo.    64  pp. 
Bartholomew's  Sketches  from  Nature.    In  five  numbers. 


BARTHOLOMEW'S  DRAWING-BOOKS. 

This  series  of  Drawing- Books  embraces  twelve  numbers.  Each  con- 
taining twelve  plates,  executed  in  the  highest  style  of  lithographic  art, 
and  twenty-four  pages  of  drawing-paper  of  superior  quality. 

Instruction  relating  to  the  examples  is  given  on  the  covers  of  the 
books. 

The  subject-matter  of  each  book  is  as  follows  :  — 

No.  i.  —  Horizontal  and  vertical  lines,  together  with  plane  figures 
and  ornamental  forms  composed  of  these  lines. 

No.  2.  —  Inclined  lines  and  ornamental  forms  composed  of  horizon- 
tal, vertical,  and  inclined  lines,  curved  lines,  circles,  and  ornamental 
forms  composed  of  curved  lines. 

No.  3.  —  Initiatory  lessons  in  Perspective  ;  The  method  of  drawing 
from  objects  explained  ;  The  Laws  of  Light ;  Shade  and  Shadow  pre- 
sented. 

No.    4.  —  Advanced  lessons  on  the  subjects  presented  in  No.  3. 

No.     5.  —  Lessons  in  drawing  Fruit  and  Flowers. 

No.    6.  —  Initiatory  lessons  on  Foreground  and  Foliage. 

No.     7.  —  Lessons  on  Landscape-drawing. 

No.     8.  —  Marine  Views  and  Landscape. 

No.    9." —  Initiatory  lessons  on  Animal-drawing. 

No.  10.  —  Advanced  lessons  on  Animal-drawing. 

No.  n.  —  Initiatory  lessons  on  Figure-drawing. 

No.  12.  — Advanced  lessons  on  Figure-drawing. 
12 


Bartholomew 's  Drawing  Series. 


The  object  aimed  at  in  the  first  four  numbers  is  to  give  to  pupils  in  our 
public  schools  that  facility  of  hand,  that  discipline  of  eye,  and  that 
knowledge  of  the  principles  of  drawing  which  all  should  possess.  The 
remaining  numbers  of  this  series  are  intended  for  the  use  of  those  who 
have  the  time  and  opportunity  to  pursue  the  study  further. 

For  the  assistance  of  teachers  the  author  has  prepared  a  series  of 
Manuals,  called  "  The  Teacher's  Guide."  With  the  aid  afforded  by 
these  manuals,  any  good  teacher  may  guide  a  class  to  successful  results. 


BARTHOLOMEW'S    PRIMARY-SCHOOL  SLATE, 

WITH   A   SERIES   OF   PROGRESSIVE   LESSONS    IN    WRITING  AND 
DRAWfNG. 

This  is  one  of  the  most  simple,  practical,  and  useful  arrangements 
which  ingenuity  has  yet  devised  in  the  way  of  a  slate  for  primary 
schools. 

BARTHOLOMEW'S    PROGRESSIVE    PICTUR- 
ESQUE  DRAWING-CARDS. 

Four  sets,  or  numbers.  Each  containing  twelve  cards,  accompanied 
with  instructions. 

These  cards  afford  pleasing  subjects  for  drawing.  The  object  aimed 
at,  however,  is  not  so  much  to  teach  the  art  of  drawing,  as  it  is  to 
nourish  in  the  child  a  love  for  it,  afford  him  a  source  of  innocent 
occupation  and  amusement,  and  lead  him  to  observe  nature. 

No.  i.  —  Elevations  of  familiar  objects,  principally  of  buildings  ; 
subjects  simple  in  outline  and  treatment 

No.  2.  —  Buildings  and  familiar  objects  in  perspective. 

No.  3.  —  Buildings  and  foliage. 

No.  4.  —  Landscape. 

BARTHOLOMEW'S  LINEAR  PERSPECTIVE. 

I    VOLUME,    IN   WHICH    THIS    SUBJECT    IS  SCIENTIFICALLY   TREATED. 


BARTHOLOMEW'S  SKETCHES  FROM  NATURE. 

IN    FIVE   NUMBERS,    TAPER   COVERS,    EACH    NUMBER   CONTAINS 
FOUR   PLATES,    II  BY  14. 

Accurate  copies  of  the  author's  pencil  sketches.  Affording  a  pleas- 
ing variety  of  subjects,  remarkable  for  simplicity  and  power.  To 
those  who  have  had  some  little  practice  in  landscape  drawing,  these 
sketches  will  prove  exceedingly  useful  as  subjects'  for  further  study. 

BARTHOLOMEW'S   DRAWING-CARDS,   FOR 
BLACKBOARD   USE. 


13 


PAYSON,  WJHTON,   &  SCIUBNER'S 
NATIONAL  SYSTEM 

OF 

PENMANSHIP 

IS  USED  ALMOST  EXCLUSIVELY  IN 

New  England,  the  British  Provinces,  the  Southern  States, 
and  the  States  of  the  Pacific  Coast. 


It  has  been  re-engraved  and  republished  in  England,  Scotland, 
and  Canada.  Large  sales  have  been  made  in  the  West  Indies,  Sand- 
wich Islands,  and  New  Mexico. 

The  Publishers  of  this  popular  system 'of  penmanship  have  spared 
no  pains  to  make  it  worthy  of  the  leading  position  which  it  holds  in 
the  country. 

We  have  endeavored  to  combine  every  practical  point  of  excellence 
which  can  be  secured  through  eminent  sagacity  and  ingenuity  of  au- 
thorship, and  the  most  artistic  skill  and  precision  in  the  mechanical 
execution  of  these  books  and  charts. 

Our  effort  to  advance  the  standard  of  good  penmanship  is  shown  in 
the  progressive  character  of  our  succeeding  editions  ;  the  favorable 
reception  which  the  books  have  met  with,  and  the  enviable  reputation 
which  they  now  enjoy,  is  regarded  a  reasonable  subject  of  congratula- 
tion. 

The  first  four  numbers  have  been  recently  revised,  rearranged,  and 
superbly  engraved. 

The  system  comprises  three  distinct  series,  — 

COMMON  SCHOOL  SEKIES,  Nos.  i,  2,  3,  4,  5,  and  6. 
BUSINESS  SERIES,  Nos.  7,  11,  and  12. 

LADIES*   SERIES,  Nos.  8,  9,  and  10. 
14 


HANAFORD  &  PAYSON'S 

BOOK-KEEPING, 

COMPRISED    IN   THREE   BOOKS, 

BY 

L.  B.  HANAFORD,  A.  M.,  AND  J.  W.  PAYSON. 


SINGLE  ENTRY  —  Common  School  Edition,  with  Blanks. 
DOUBLE  AND  SINGLE  ENTRY  —  High  School  Edition,  with 
Blanks. 

ACADEMIC  EDITION  — with  Blanks. 

This  work  completely  meets  the  wants  of  the  older  pupils  in  our 
Common  Schools  and  Academies ;  it  has  met  a  hearty  reception,  and 
given  universal  satisfaction. 


CROSBY'S    GREEK    SERIES. 

The  series  comprises  the  following  books :  — 

GREEK   LESSONS.     Consisting  of  selections  from  Xenophon's 
Anabasis,  with  Directions   for  the  Study  of  the  Grammar,  Notes, 
-  Exercises  in  Translation  from  English  into  Greek,  and  a  Vocabu- 
lary.    By  ALPHEUS  CROSBY.     i2mo.     Price,  $  i.oo. 

XENOPHON'S  ANABASIS.  Revised  Edition.  A  Narrative 
of  the  Expedition  of  Cyrus  the  Younger,  and  of  the  Retreat  of  the 
Ten  Thousand.  By  XENOPHON  of  Athens.  Edited  by  ALPHEUS 
CROSBY.  I2mo.  Price,  $  1.25. 

GREEK  TABLES.  For  the  Use  of  Students.  By  ALPHEUS 
CROSBY.  I2mo.  Price,  62  cents. 

A  GRAMMAR  OF  THE  GREEK  LANGUAGE.  Re- 

vised  Edition.     By  ALPHEUS  CROSBY,  late  Professor  of  the  Greek 
Language  and  Literature  in  Dartmouth  College.      I2mo.     Price, 

$  1-75- 

15 


PAYSON,    DUNTON,    &    SCRIBNER'S 

STEEL   PENS. 


Having  added  ONF  NEW  PEN  to  our  series,  we  feel  that  it  embraces 
variety  enough  to  meet  all  wants  and  suit  all  tastes. 

These  PENS  are  made  expressly  for  us  by  the  best  manufacturers  in 
England  and  America ;  and  in  quality  of  material,  finish  of  points, 
easy  action,  and  durability  are  unsurpassed  by  any  in  the  market. 

No.  333.    EXTRA   FINE. 

No.  44,5.     THE  NATIONAL  PEN. 

No.  7.     THE  BUSINESS  PEN. 

No.  8.     THE  LADIES'  PEN. 

No.  111.     COMMERCIAL  PEN. 

No.  117.     THE  EXCELSIOR  PEN. 

These  Pens  are  neatly  put  up  in  gross  and  quarter-gross  boxes. 
Sample  card  of  six  pens  (one  of  each  kind)  sent  to  any  address  on 
receipt  of  ten  cents. 

These  Sample  Cards  offered  to  the  trade  at  80  cents  per  dozen. 


RICHARD'S    LATIN    LESSONS. 
SAWYER'S    LATIN    PRIMER. 

WILSON'S    TREATISE    ON   ENGLISH   PUNCTU- 
ATION. 

BASCOM'S    WORKS. 

1.  ^Esthetics ;  or,  The  Science  of  Beauty. 

2.  Philosophy  of  Rhetoric* 

CHAMPLIN'S    WORKS. 

1.  Text-Book  in  Intellectual  Philosophy. 

2.  First  Principles  of  Ethics. 


WOOLWORTH,  AINS WORTH,  &  CO.,  PUBLISHERS, 

NEW    YORK. 

16 


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